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The impact of direct aperture optimization on plan quality and efficiency in complex head and neck IMRT

The impact of direct aperture optimization on plan quality and efficiency in complex head and... Background: Conventional step&shoot intensity modulated radio therapy (IMRT) approaches potentially lead to treatment plans with high numbers of segments and monitor units (MU) and, therefore, could be time consuming at the linear accelerator. Direct optimization methods are able to reduce the complexity without degrading the quality of the plan. The aim of this study is the evaluation of different IMRT approaches at standardized conditions for head and neck tumors. Method: For 27 patients with carcinomas in the head and neck region a planning study with a 2-step-IMRT system (KonRad), a direct optimization system (Panther DAO) and a mixture of both approaches (MasterPlan DSS) was created. In order to avoid different prescription doses for boost volumes a simple standardization was realized. The dose was downscaled to 50 Gy to the planning target volume (PTV) which included the primary tumor as well as the bilateral lymphatic drainage (cervical and supraclavicular). Dose restrictions for the organs at risk (OAR) were downscaled to this prescription from high dose concepts up to 72 Gy. Those limits were defined as planning objectives while reaching definable PTV coverage with a standardized field setup. The parameters were evaluated from the corresponding dose volume histogram (DVH). Special attention was paid to the efficiency of the method, measured by means of calculated MU and required segments. Statistical tests of significance were applied to quantify the differences between the evaluated systems. Results: PTV coverage for all systems in terms of V and V fell short of the requested 100% and 95%, 90% 95% respectively, but were still acceptable (range: 98.7% to 99.1% and 94.2% to 94.7%). Overall for OAR sparing and the burden of healthy tissue with low doses no technique was superior for all evaluated parameters. Differences were found for the number of segments where the direct optimization systems generated less segments. Lowest average numbers of MU were 308 by Panther DAO calculated for 2 Gy fractions. Based on these findings the treatment time at the linear accelerator is the lowest for Panther DAO. Conclusions: All IMRT approaches implemented in the different treatment planning systems (TPS) generated clinically acceptable and comparable plans. No superior system in terms of PTV coverage and OAR sparing was found. Major differences in efficiency of the method in terms of calculated MU and treatment times were found. Keywords: head-and-neck, planning study, 2-step-IMRT, direct aperture optimization * Correspondence: Marcello.Sabatino@radiologische-allianz.de Department of Radiation Therapy and Radiooncology, Radiologische Allianz Hamburg, Hamburg, Germany Full list of author information is available at the end of the article © 2012 Sabatino et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Sabatino et al. Radiation Oncology 2012, 7:7 Page 2 of 8 http://www.ro-journal.com/content/7/1/7 of the maximum are allowed to escape from local minima Background in the beginning of the process. The variation of step size The exploration and evaluation of the clinical role of is generated from a Cauchy distribution. First large step IMRT software from different vendors for complex treat- sizes are possible and then rapid changes in step size are ment planning in head and neck tumors has been the allowed [14]. subject of numerous studies [1-9]. These studies com- The user must specify the number of apertures to be pared static step&shoot and sliding windows IMRT with optimized for each field. Optionally, a minimum number dynamic rotational IMRT with regards to PTV coverage of MU are determined for an aperture, which may not and sparing of OAR. Because of the complexity of treat- be violated. These conditions are, in addition to the ment delivery, more attention was paid to the efficiency dose prescriptions, taken into account directly during of the method to reach the desired dose distribution. The optimization. During the optimization process, the pro- aim was to reduce the complexity without any concession gress is calculated with a Pencil Beam algorithm (PB) to the quality of a plan. A problem which can occur is and visualized with an objective function value and a that treatment time takes much longer for traditional dose volume histogram (DVH). If the result is satisfac- IMRT techniques as compared to conformal radiotherapy tory, the final dose calculation is performed with a col- techniques. The number of MU and segments are also a lapsed cone convolution (CCC). matter of concern. In Oncentra MasterPlan version 3.3 (Nucletron BV, Particularly direct optimization systems achieved good Veenendaal, Netherlands) a Direct Step and Shoot results compared with the conventional 2-step IMRT (first License (DSS) is available to generate fluence modulated step: calculation of fluence modulated distribution, second fields with a direct optimization approach. A determinis- step: conversion into a deliverable sequence of segments) tic gradient algorithm minimizes the objective function. in the above mentioned aspects [3,5,9-11]. Furthermore, In order to find a good starting point for the direct opti- these technique might be favorable concerning radiation mization procedure, fluence profiles are calculated as for protection aspects for the patient due to reduced collima- the 2-step approach within a user defined number of tor head leakage and possibly reduced scattered radiation iterations. Until the predefined iterations are reached, the from the patient [12]. conversion is administered in segments. The sequencer This study compares 2-step and directly optimized produces approximately the number of segments pre- IMRT approaches. The planning study was carried out viously defined by the user. These segments are then retrospective on 27 previously contoured and clinically incorporated into the optimization process and are opti- treated head and neck cases. Plans were calculated at standardized planning conditions in terms of same linear mized directly in all subsequent iterations. From that point on only the leaf placement and weighting of the accelerator, gantry angles and planning objectives for apertures is varied. The number of segments as well as PTV coverage and OAR sparing. the position of the jaws remains unchanged. For the seg- mentation process a minimal number of MU per seg- Methods ment and minimum field openings are to be set. The Treatment planning systems dose calculation during optimization is performed with a IMRT plans were calculated with Panther DAO version simplified algorithm. In the conversion step and for the 4.71 (Concord, CA, USA). The optimization was done final calculation a full CCC is used [15]. with the direct aperture optimization (Panther DAO) KonRad is a 2-step inverse planning tool from Sie- approach. For this purpose a leaf is selected randomly and mens AG (Erlangen, Germany). All plans used for this a Gaussian distribution determines step length and direc- study were created with version 2.2.23. The optimization tion of the leaf. Simultaneously, the weighting of the aper- is carried out with a Gradient-Newton method, which ture is optimized. A stochastic fast-simulated annealing optimizes fluence distributions. A leaf sequencer trans- algorithm is minimizing the objective function and has lates the fluence distribution into segments. The number two possible options to overcome local minima. In order of produced segments depends mainly on two factors: to favor “tunneling”, this algorithm uses iteration-step the chosen number of steps of intensity levels and the sizes derived from Gaussian distributions. At the begin- usage of a median filter. If the intensity levels are set to ning, large steps are possible, which will be reduced during a high value, the leaf sequencer has the possibility to the optimization process [11]. Furthermore with the pro- approach the calculated distribution with more seg- gress of optimization, the step size of “running up” the ments, which, in general, leads to a better approxima- objective function is changed. The probability of accepting tion of the initial fluence. Another influence is the usage large steps “uphill” is controlled by the dynamic para- of a median filter which smoothes strong gradients meters of the temperature [13]. Transferred to the sto- within the intensity distribution. The impact depends on chastic optimization algorithm, large steps in the direction Sabatino et al. Radiation Oncology 2012, 7:7 Page 3 of 8 http://www.ro-journal.com/content/7/1/7 the dimension of the filter [6]. For dose calculation a PB Different to the other systems, Panther DAO allowes algorithm is used. fluence modulation with fixed jaws so the modulation is done by the leaves themselves. Patient population and dose prescription Planning objectives were formulated to avoid differences The study enrolled 27 patients with carcinomas in the in the optimization weighting factors and the way these head and neck region. Depending on the location of the are interpreted by the planning software. The primary goal tumor, the PTV included the primary tumor of the oro-, was to treat the PTV with a minimum of 95% and a maxi- hypo-, nasopharynx or larynx and the bilateral cervical mum of 107% of the prescribed dose [18], which should and supraclavicular lymphatic drainage (mean target ideally lead to a median dose of 50 Gy. No dose normali- volume:1209 ± 281 cm ). In the immediate vicinity the zation took place. Furthermore, the volume of the PTV relevant OAR are the spinal cord, the brain stem (depend- which received 90% and 95% of the prescribed dose (V / 90% ing on the tumor location and extent of PTV) and the par- V ) should reach 100% respective 95% of the target 95% otid glands. A total dose of 50 Gy was delivered to the dose. Secondary objectives were the above mentioned PTV including the lymphatic drainage area and boost irra- dose limits to the OAR, provided that the targets goals diation to a maximum of 72 Gy could follow. This might were met satisfactorily. Consistent support structures were result in a wide variation of boost contours (volume, posi- created for all systems in order to steer the optimization tion, prescribed dose), for which a large number of plans and to avoid overdose outside the PTV. These structures with different dose prescriptions would have to be created. will be integrated into the planning process, but no plan- In order to achieve comparability between the systems a ning objectives, in terms of V should be below a certain xGy simple standardization was applied. Plans for the low-dose percentage, were defined for these volumes. Further target area were created and set to a fractionation of 2 Gy attempts were made to achieve the planning specifications in 25 fractions. As the total dose was kept to 50 Gy (and with as few segments as possible. not to 72 Gy as it would be in the curative approach) the maximum dose affecting the spinal cord, parotid glands Evaluation methods and brainstem was reduced proportionally. Thus, the max- The plans were compared and analyzed using DVH. For imum dose to the spinal cord and brainstem was kept all systems, except KonRad, the data are analyzed in each below 30 Gy, and that to the parotid glands below 19 Gy program’s own analysis tool. Since KonRad has no output [16]. Healthy tissue is defined as outer contour of the for specific D -and V -values, the CT scans, dose and x% yGy patient subtracted by the PTV. This volume is limited in volume structures were exported into MasterPlan, where craniocaudal direction. the plans were further analyzed. No recalculation of dose took place. Planning methodology Because of the fact that the dose calculation algo- Thestandardfield setupusedforallplans consistedof rithms of the treatment planning systems (TPS) have seven static 6 MV photon fields with gantry angles of 0°, difficulties and different approaches to model the build- 52°, 104°, 156°, 204°, 256°, 308° with a dose rate of 300 up effect, the PTV was retracted 3 mm from the outline MU/min. All plans were calculated for a clinical used [19].ThismodifiedPTV wasusedfor furtheranalysis. Artiste Linac (Siemens AG, Erlangen, Germany) Maximum doses were included in the assessment by the equipped with a 160 Leaf MLC. Further details and dosi- parameter of V . The homogeneity index (HI =[D - 107% 2% metric characteristics were investigated by Tacke et al. D ]/D ) reflects how steep the dose drop off 98% prescription [17]. The planner tried to keep the number of segments in the PTV is. A smaller HI indicates a more homoge- as low as possible while fulfilling the planning goals. The nous dose distribution. maximum segment number allowed for DSS plans were Maximum doses in serial OAR are reflected on D .The 2% kept to a minimum while reaching the planning objec- dose to the major salivary glands was recorded at the med- tives. In order to minimize the number of segments for ian dose. Low-dose exposure of healthy tissue was KonRad the intensity levels and used median filter were reported as the volume which receives 5 Gy (V )and 5Gy varied. Since the number of segments per beam direction 10 Gy (V ). For all plans the treatment times were 10 Gy is fixed at Panther DAO due to user definitions, the plan- measured from the beginning of the first field until the ner was allowed to apply split beams to provide addi- end of the last segment. The efficiency of the IMRT tional degrees of freedom. This procedure is doubling the method was derived from the calculated MU and required beam from one direction and therefore doubling the segments. number of segments from this particular direction. The To quantify the differences of parameters between two cut off for all systems per segment was set to 5 MU with systems a test of significance is required. Since the mea- a minimum field size of 4 cm . surements were collected for each planning system for Sabatino et al. Radiation Oncology 2012, 7:7 Page 4 of 8 http://www.ro-journal.com/content/7/1/7 the same collective, a two-sided, paired student t-test was and has a low standard deviation (0.01). However, statis- used. Statistical significance was defined for p-values < tical significant differences for the HI were found for all 0.05. A Kolmogorov-Smirnov-test was applied before, planning systems. The p-values are < 0.001 for DSS vs. whereby the parameters were tested with regard to a nor- Panther DAO and KonRad, and Panther DAO vs. mal distribution. KonRad. All results of this study are reported as averages of the entire patient cohort and the appropriate standard Organs at risk and low dose exposure deviation. The results of DVH analysis for the OAR are listed in Table 2. Only KonRad met the planning objectives for Results the serial riskstructures. The average maximum doses to Dose-coverage for PTV the spinal cord are 30.0 Gy (KonRad), 30.6 Gy (DSS) All IMRT systems reach satisfactory and comparable and 31.5 Gy (Panther DAO). KonRad is best in sparing results for the dose in the PTV. The prescribed median the brainstem with 26.8 Gy followed by DSS (27.2 Gy) dose of 50 Gy is achieved in all cases. Figure 1 shows and Panther DAO (30.9 Gy). Statistical significant differ- theaverage DVHfor the entirepatient cohort forthe ences are observed between Panther DAO and the two three TPS. other TPS. The prescribed high-dose objectives for V and V The planning objective for major parotid gland sparing 90% 95% come close to the requested aim (100% respective 95%). was difficult to achieve. The median dose varied The volume which receives more than 107% of the pre- between 19.7 Gy (DSS) to 21.0 Gy (Panther DAO) and scription dose is lowest for DSS (0.3%) followed by 21.3 Gy (KonRad) (see Table 2). Panther DAO (0.7%) and KonRad (1.2%). Table 1 shows The exposure of healthy tissue to doses below 5 Gy and the results for the PTV according to the DVH analysis. 10 Gy is presented in Table 2. Statistical significance are The HI is within a close range for all tested systems observed for all values and planning systems except for (0.12 for DSS; 0.13 for Panther DAO; 0.14 for KonRad) V between KonRad and DSS. The exposure at this 5Gy DAO DSS KonRad 30 40 50 Dose [Gy] Figure 1 Dose volume histogramm for the PTV for the three tested IMRT-systems.Shown is theaverage DVHfor thewhole patient cohort, different colours denoting the different TPS. Volume [%] Sabatino et al. Radiation Oncology 2012, 7:7 Page 5 of 8 http://www.ro-journal.com/content/7/1/7 Table 1 Dosimetric results for the PTV from 2-step and Table 3 Average MU and treatment time for the three direct optimized IMRT for the tested TPS. different optimization systems. Parameter DAO KonRad DSS p Parameter DSS KonRad DAO Median [Gy] 50.3 ± 0.1 50.3 ± 0.1 50.3 ± 0.1 - MU 807 ± 110 564 ± 78 308 ± 21 V [%] 0.7 ± 0.4 1.2 ± 0.6 0.3 ± 0.5 a, b, c reduction - 30% 62% 107% V [%] 94.7 ± 0.8 94.2 ± 1.3 94.7 ± 1.1 a treatment time [min] 10.5 ± 1.2 9.75 ± 1.2 7.0 ± 0.9 95% V [%] 99.1 ± 0.4 98.7 ± 1.1 99.1 ± 0.4 - reduction [min] - 0.75 3.5 90% HI [-] 0.13 ± 0.01 0.14 ± 0.01 0.12 ± 0.01 a, b, c Given are the calculated average MU for a 2 Gy fraction, the average treatment time and the respective reduction to the highest values (DSS). Given is the median dose, the volume which receives 107%, 95% and 90% of the prescribed dose as well as the homogeneity index. Statistical significance (p < 0,05): a: Panther DAO vs KonRad, b: Panther DAO In terms of PTV coverage and OAR sparing all sys- vs DSS, c: KonRad vs DSS, tems reach satisfactory and clinically acceptable results, even though some statistical significant differences can dose level is lowest for Panther DAO followed by Kon- be observed. But the clinical relevance at this level is Rad and DSS. questionable. Similar conclusions could be stated for the risk structures spinal cord, brainstem and parotid Evaluation of efficiency glands. Panther DAO is violating the planning objectives The number of MU for a 2 Gy fraction resulted in 308 ± but if one projects this percentage dose reduction to a 21 MU for Panther DAO, 564 ± 78 MU for KonRad and totalprescriptiondoseof72Gy, sparingofthe spinal 807 ± 101 MU for DSS. Compared to DSS the percen- cord is possible. Greater variations were observed in the tages of MU reduction are 30% (KonRad) and 62% efficiency of the intensity modulation. Compared to the (Panther DAO). The results are shown in table 3 and as 2-step approach the direct optimization algorithms are boxplot diagrams for the obtained MU in Figure 2. In a able to decrease the number of segments. DSS generates similar way the required segments are shown in Figure 3. 22% and Panther DAO 37% less segments than KonRad. On average 43 ± 9 segments are needed for Panther One aim was to keep the number of segments per plan DAO, 53 ± 8 segments for DSS and 68 ± 7 segments for as low as possible while reaching the planning goals for KonRad. PTV coverage and OAR sparing. One reason for the Treatment times were measured for all patients and efficiency of Panther DAO is the possibility to create a planning systems. The longest average treatment time was highly modulated field with few segments. With 5 seg- 10.5 ± 1.2 min for DSS. For the KonRad system 9.75 ± 1.2 ments per beam, 31 intensity levels (intensity levels = min were observed and 7.0 ± 0.9 min for Panther DAO. 2 -1) can be generated [11]. Concerning calculated MU Figure 4 shows the correlation between the MU and for a 2 Gy fraction the purely direct optimization system the number of segments. For DSS, the relationship has the lowest values. The reduction for Panther DAO is between these variables is most pronounced (R = 62% and for KonRad 30% compared to DSS. 0,668). A moderate increase is recorded with the Kon- Statistical differences were found for the low dose Rad (R = 0.335) system. Theoretically 80 segments with exposure. Two reasons could be responsible for this Panther DAO (R = 0.541) would not exceed 400 MU. finding: chosen gantry angles and dose calculation algo- rithms. In this case the large deviations could not be Discussion explainedbythegantryangles sincetheywereall the This study compares step&shoot IMRT for head and same for all plans and systems. neck tumors at standardized conditions with special The presumption is that the differences occur due to attention to their different optimization approaches. less MU. But since the TPS calculations accuracy for Table 2 Dosimetric results for spinal cord, brainstem, summed parotid glands and healthy tissue. Organ Parameter DAO KonRad DSS p Spinal cord D [Gy] 31.5 ± 1.1 30.0 ± 1.6 30.6 ± 1.4 a, b 2% Brainstem D [Gy] 31.0 ± 2.2 26.8 ± 2.6 27.2 ± 2.6 a, b 2% Summed Parotids D [Gy] 21.0 ± 1.8 21.3 ± 2.1 19.7 ± 1.8 b, c Median Healthy Tissue V [%] 66.7 ± 5.4 73.5 ± 5.4 73.6 ± 4.2 a, b 5Gy Healthy Tissue V [%] 53.8 ± 5.8 57.8 ± 5.2 59.4 ± 3.8 a, b, c 10 Gy Given is the dose in Gy to two percent of the volume of the spinal cord and brainstem, median dose to the parotid glands and the volume of healthy tissue receiving more than 5 Gy respective 10 Gy. Statistical significance (p < 0,05); a: Panther DAO vs KonRad, b: Panther DAO vs DSS, c: KonRad vs DSS Sabatino et al. Radiation Oncology 2012, 7:7 Page 6 of 8 http://www.ro-journal.com/content/7/1/7 Figure 2 Boxplot diagram of monitorunit distribution calculated from the three tested IMRT-systems. The median is shown as a blue st rd line, maximum and minimum in red, and 1 and 3 quartile as thin black lines. low doses is reduced, an experimental measurement (R = 0.541) can be expected. Rather a further increase could verify these findings. of segments could result in potentially dosimetric For Panther DAO - even if highly rising numbers of unstable conditions, as the number of MU per segment segments occurred - only a moderate increase of MU may be too small. A stronger correlation was found for Figure 3 Boxplot diagram of required segments for the three tested IMRT-systems. The median is shown as a blue line, maximum and st rd minimum in red, and 1 and 3 quartile as thin black lines. Sabatino et al. Radiation Oncology 2012, 7:7 Page 7 of 8 http://www.ro-journal.com/content/7/1/7 Figure 4 Linear correlation between number of segments and MU. The MU are given according to the number of segments for each TPS. The quality of the correlation is given with R . Lines indicate fitted linear correlation and the corresponding equation is given. DSS (R = 0.669). A larger variation of the pair of values less segments. The exposure time is reduced by 29%. Dobler et al. evaluated the effects of DSS compared to occur in KonRad (R = 0.335). Theprimary focusofthisstudy is the MU efficiency the 2-step approach of MasterPlan [3]. The study was of the compared optimization algorithm. This is owed conducted with 10 patients with a hypopharyngeal carci- to the increasing number of MU in IMRT in compari- noma with the same field arrangement and fraction dose son to 3D-conformal radio therapy (3DCRT) which as in this study. A reduction in MU from 1151 to 901 could increase the risk of radiation induced secondary was found in favor of the direct optimization procedure. malignancies due to scattered radiation. Panther DAO The required average segment number of 77 is the same plans could decrease the amount of scatter radiation ori- for both approaches. In a further planning study con- ginating from the collimator head. Hall pointed out the cerning head and neck tumors with integrated boost, need for protection of patients from scattered radiation Wiezorek et al. compared static and rotational IMRT in IMRT-treatments [12]. He reported a potential and Tomotherapy as well as different optimization algo- increase of radiation-induced cancer due to larger total- rithms [9]. Normalized MU were found to be lowest for body doses caused by leakage radiation. Considering this Panther DAO. aspect, MU reduced plans with comparable quality The low number of MU and segments is the main should be preferred, especially for pediatric cases or dis- reason for the shortest treatment times for Panther eases of young adults and adolescents with highly cur- DAO. On average, the amount of time saved is 3.5 min able concepts. (DSS) and 2.8 min (KonRad). These time savings could The reported reductions are in agreement with pub- be used for image guidance. In addition to that it is lished studies. Jones et al. compared 2-step IMRT with advantageous for intrafractional movement of the organs directly optimized IMRT plans using the Pinnacle and for the comfort of the patient. A reason for these DMPO in a planning study for head and neck tumors time savings is the above mentioned creation of fluence [5]. For this system an approach for the direct optimiza- modulation. Within a field the modulation is done by the fast leaves while the slower jaws are fixed to one tion, which is similar with DSS, is implemented. It is reported that DMPO requires 42% less MU and 35% position. Sabatino et al. Radiation Oncology 2012, 7:7 Page 8 of 8 http://www.ro-journal.com/content/7/1/7 sequencing in IMRT of hypopharyngeal carcinoma. Radiat Oncol 2007, In the planning study by Wiezoreck et. al the exposure 2:33. of healthy tissue to low doses were also evaluated. The 4. Fogliata A, Bolsi A, Cozzi L: Comparative analysis of intensity modulation low doses to healthy tissue were found to be highest for inverse planning modules of three commercial treatment planning systems applied to head and neck tumour model. Radiother Oncol 2003, the Panther DAO system [9]. These findings differ from 66:29-40. the results of our study. Differences may occur due to 5. Jones S, Williams M: Clinical evaluation of direct aperture optimization different approaches to calculate the values for the low when applied to head-and-neck IMRT. Med Dosim 2008, 33:86-92. 6. Reitz B, Miften M: Comparison of the KonRad IMRT and XiO treatment dose exposure. In this study the external was subtracted planning systems. J Appl Clin Med Phys 2008, 9:2770. by the PTV and the extent of this new volume limited 7. Vanetti E, Clivio A, Nicolini G, et al: Volumetric modulated arc to 3 cm in craniocaudal direction from the PTV. This radiotherapy for carcinomas of the oro-pharynx, hypo-pharynx and larynx: a treatment planning comparison with fixed field IMRT. Radiother was done because of a limited calculation matrix in Oncol 2009, 92:111-117. KonRad. 8. Verbakel WF, Cuijpers JP, Hoffmans D, Bieker M, Slotman BJ, Senan S: Another reason could be the number of chosen gantry Volumetric intensity-modulated arc therapy vs. conventional IMRT in head-and-neck cancer: a comparative planning and dosimetric study. Int angles. In the study of Wiezoreck et al. eleven beam J Radiat Oncol Biol Phys 2009, 74:252-259. directions were taken for Panther DAO. 9. Wiezorek T, Brachwitz T, Georg D, et al: Rotational IMRT techniques compared to fixed gantry IMRT and tomotherapy: multi-institutional planning study for head-and-neck cases. Radiat Oncol 6:20. Conclusions 10. Broderick M, Leech M, Coffey M: Direct aperture optimization as a means All IMRT systems are able to calculate acceptable plans of reducing the complexity of Intensity Modulated Radiation Therapy in terms of PTV coverage and OAR sparing. Main differ- plans. Radiat Oncol 2009, 4:8. 11. Shepard DM, Earl MA, Li XA, Naqvi S, Yu C: Direct aperture optimization: a ences are observed in the efficiency of the fluence modu- turnkey solution for step-and-shoot IMRT. Med Phys 2002, 29:1007-1018. lation. Based on the results of published literature and 12. Hall EJ: Intensity-modulated radiation therapy, protons, and the risk of the results of this study, a further reduction of plan com- second cancers. Int J Radiat Oncol Biol Phys 2006, 65:1-7. 13. Webb S: The physical basis of IMRT and inverse planning. Br J Radiol plexity can be stated for the purely direct-optimizing 2003, 76:678-689. Panther DAO system in IMRT planning of complex head 14. Oelfke U, Nill S, Wilkens JJ: Physical Optimization.Edited by: Bortfeld TR, and neck cases. The reduced number of segments and Schmidt-Ullrich R, De Neve W, Wazer DE. Image-guided IMRT, Berlin Heidelberg: Springer; 2006:31-46. MU should lead to less leakage radiation from the colli- 15. Hardemark B, Liander A, Rehbinder H, Löf J: Direct machine parameter mator head. If and how much these reductions lead to optimization with RayMachine in Pinnacle. Ray-Search White Paper 2003. less peripheral doses should be verified by experimental 16. Marks LB, Yorke ED, Jackson A, et al: Use of normal tissue complication probability models in the clinic. Int J Radiat Oncol Biol Phys 76:S10-19. measurements as performed by Wiezorek et al. [20]. 17. Tacke MB, Nill S, Haring P, Oelfke U: 6 MV dosimetric characterization of the 160 MLC, the new Siemens multileaf collimator. Med Phys 2008, 35:1634-1642. Author details 18. ICRU Report 50-Prescribing, recording and reporting photon beam Department of Radiation Therapy and Radiooncology, Radiologische Allianz therapy. International Commission on Radiation Units and Measurements Hamburg, Hamburg, Germany. Institute of Radiation Protection and Medical Physics,Technische Hochschule Mittelhessen, Giessen, Germany. 19. Chung H, Jin H, Dempsey JF, et al: Evaluation of surface and build-up region dose for intensity-modulated radiation therapy in head and neck Authors’ contributions cancer. Med Phys 2005, 32:2682-2689. MS and MK contributed significantly to study design and concept. MS was 20. Wiezorek T, Schwahofer A, Schubert K: The influence of different IMRT responsible for treatment planning. Data analysis and interpretation of techniques on the peripheral dose: a comparison between sMLM-IMRT results were performed by MS and MK. Corrections and/or improvements and helical tomotherapy. Strahlenther Onkol 2009, 185:696-702. were suggested by MK, KZ and FW. FW was responsible for clinical evaluation of the treatment plans. All authors read and approved the final doi:10.1186/1748-717X-7-7 manuscript. Cite this article as: Sabatino et al.: The impact of direct aperture optimization on plan quality and efficiency in complex head and neck Competing interests IMRT. Radiation Oncology 2012 7:7. The Department of Radiation Therapy and Radiooncology of the Radiologische Allianz Hamburg receives research grants from Prowess Inc., Siemens AG Healthcare sector and Nucletron BV. Received: 30 August 2011 Accepted: 23 January 2012 Submit your next manuscript to BioMed Central Published: 23 January 2012 and take full advantage of: References • Convenient online submission 1. Bertelsen A, Hansen CR, Johansen J, Brink C: Single Arc Volumetric Modulated Arc Therapy of head and neck cancer. Radiother Oncol 2010, • Thorough peer review 95:142-148. • No space constraints or color figure charges 2. Cozzi L, Fogliata A, Bolsi A, Nicolini G, Bernier J: Three-dimensional • Immediate publication on acceptance conformal vs. intensity-modulated radiotherapy in head-and-neck cancer patients: comparative analysis of dosimetric and technical parameters. • Inclusion in PubMed, CAS, Scopus and Google Scholar Int J Radiat Oncol Biol Phys 2004, 58:617-624. • Research which is freely available for redistribution 3. Dobler B, Pohl F, Bogner L, Koelbl O: Comparison of direct machine parameter optimization versus fluence optimization with sequential Submit your manuscript at www.biomedcentral.com/submit http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Oncology Springer Journals

The impact of direct aperture optimization on plan quality and efficiency in complex head and neck IMRT

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Copyright © 2012 by Sabatino et al; licensee BioMed Central Ltd.
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Medicine & Public Health; Oncology; Radiotherapy
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Abstract

Background: Conventional step&shoot intensity modulated radio therapy (IMRT) approaches potentially lead to treatment plans with high numbers of segments and monitor units (MU) and, therefore, could be time consuming at the linear accelerator. Direct optimization methods are able to reduce the complexity without degrading the quality of the plan. The aim of this study is the evaluation of different IMRT approaches at standardized conditions for head and neck tumors. Method: For 27 patients with carcinomas in the head and neck region a planning study with a 2-step-IMRT system (KonRad), a direct optimization system (Panther DAO) and a mixture of both approaches (MasterPlan DSS) was created. In order to avoid different prescription doses for boost volumes a simple standardization was realized. The dose was downscaled to 50 Gy to the planning target volume (PTV) which included the primary tumor as well as the bilateral lymphatic drainage (cervical and supraclavicular). Dose restrictions for the organs at risk (OAR) were downscaled to this prescription from high dose concepts up to 72 Gy. Those limits were defined as planning objectives while reaching definable PTV coverage with a standardized field setup. The parameters were evaluated from the corresponding dose volume histogram (DVH). Special attention was paid to the efficiency of the method, measured by means of calculated MU and required segments. Statistical tests of significance were applied to quantify the differences between the evaluated systems. Results: PTV coverage for all systems in terms of V and V fell short of the requested 100% and 95%, 90% 95% respectively, but were still acceptable (range: 98.7% to 99.1% and 94.2% to 94.7%). Overall for OAR sparing and the burden of healthy tissue with low doses no technique was superior for all evaluated parameters. Differences were found for the number of segments where the direct optimization systems generated less segments. Lowest average numbers of MU were 308 by Panther DAO calculated for 2 Gy fractions. Based on these findings the treatment time at the linear accelerator is the lowest for Panther DAO. Conclusions: All IMRT approaches implemented in the different treatment planning systems (TPS) generated clinically acceptable and comparable plans. No superior system in terms of PTV coverage and OAR sparing was found. Major differences in efficiency of the method in terms of calculated MU and treatment times were found. Keywords: head-and-neck, planning study, 2-step-IMRT, direct aperture optimization * Correspondence: Marcello.Sabatino@radiologische-allianz.de Department of Radiation Therapy and Radiooncology, Radiologische Allianz Hamburg, Hamburg, Germany Full list of author information is available at the end of the article © 2012 Sabatino et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Sabatino et al. Radiation Oncology 2012, 7:7 Page 2 of 8 http://www.ro-journal.com/content/7/1/7 of the maximum are allowed to escape from local minima Background in the beginning of the process. The variation of step size The exploration and evaluation of the clinical role of is generated from a Cauchy distribution. First large step IMRT software from different vendors for complex treat- sizes are possible and then rapid changes in step size are ment planning in head and neck tumors has been the allowed [14]. subject of numerous studies [1-9]. These studies com- The user must specify the number of apertures to be pared static step&shoot and sliding windows IMRT with optimized for each field. Optionally, a minimum number dynamic rotational IMRT with regards to PTV coverage of MU are determined for an aperture, which may not and sparing of OAR. Because of the complexity of treat- be violated. These conditions are, in addition to the ment delivery, more attention was paid to the efficiency dose prescriptions, taken into account directly during of the method to reach the desired dose distribution. The optimization. During the optimization process, the pro- aim was to reduce the complexity without any concession gress is calculated with a Pencil Beam algorithm (PB) to the quality of a plan. A problem which can occur is and visualized with an objective function value and a that treatment time takes much longer for traditional dose volume histogram (DVH). If the result is satisfac- IMRT techniques as compared to conformal radiotherapy tory, the final dose calculation is performed with a col- techniques. The number of MU and segments are also a lapsed cone convolution (CCC). matter of concern. In Oncentra MasterPlan version 3.3 (Nucletron BV, Particularly direct optimization systems achieved good Veenendaal, Netherlands) a Direct Step and Shoot results compared with the conventional 2-step IMRT (first License (DSS) is available to generate fluence modulated step: calculation of fluence modulated distribution, second fields with a direct optimization approach. A determinis- step: conversion into a deliverable sequence of segments) tic gradient algorithm minimizes the objective function. in the above mentioned aspects [3,5,9-11]. Furthermore, In order to find a good starting point for the direct opti- these technique might be favorable concerning radiation mization procedure, fluence profiles are calculated as for protection aspects for the patient due to reduced collima- the 2-step approach within a user defined number of tor head leakage and possibly reduced scattered radiation iterations. Until the predefined iterations are reached, the from the patient [12]. conversion is administered in segments. The sequencer This study compares 2-step and directly optimized produces approximately the number of segments pre- IMRT approaches. The planning study was carried out viously defined by the user. These segments are then retrospective on 27 previously contoured and clinically incorporated into the optimization process and are opti- treated head and neck cases. Plans were calculated at standardized planning conditions in terms of same linear mized directly in all subsequent iterations. From that point on only the leaf placement and weighting of the accelerator, gantry angles and planning objectives for apertures is varied. The number of segments as well as PTV coverage and OAR sparing. the position of the jaws remains unchanged. For the seg- mentation process a minimal number of MU per seg- Methods ment and minimum field openings are to be set. The Treatment planning systems dose calculation during optimization is performed with a IMRT plans were calculated with Panther DAO version simplified algorithm. In the conversion step and for the 4.71 (Concord, CA, USA). The optimization was done final calculation a full CCC is used [15]. with the direct aperture optimization (Panther DAO) KonRad is a 2-step inverse planning tool from Sie- approach. For this purpose a leaf is selected randomly and mens AG (Erlangen, Germany). All plans used for this a Gaussian distribution determines step length and direc- study were created with version 2.2.23. The optimization tion of the leaf. Simultaneously, the weighting of the aper- is carried out with a Gradient-Newton method, which ture is optimized. A stochastic fast-simulated annealing optimizes fluence distributions. A leaf sequencer trans- algorithm is minimizing the objective function and has lates the fluence distribution into segments. The number two possible options to overcome local minima. In order of produced segments depends mainly on two factors: to favor “tunneling”, this algorithm uses iteration-step the chosen number of steps of intensity levels and the sizes derived from Gaussian distributions. At the begin- usage of a median filter. If the intensity levels are set to ning, large steps are possible, which will be reduced during a high value, the leaf sequencer has the possibility to the optimization process [11]. Furthermore with the pro- approach the calculated distribution with more seg- gress of optimization, the step size of “running up” the ments, which, in general, leads to a better approxima- objective function is changed. The probability of accepting tion of the initial fluence. Another influence is the usage large steps “uphill” is controlled by the dynamic para- of a median filter which smoothes strong gradients meters of the temperature [13]. Transferred to the sto- within the intensity distribution. The impact depends on chastic optimization algorithm, large steps in the direction Sabatino et al. Radiation Oncology 2012, 7:7 Page 3 of 8 http://www.ro-journal.com/content/7/1/7 the dimension of the filter [6]. For dose calculation a PB Different to the other systems, Panther DAO allowes algorithm is used. fluence modulation with fixed jaws so the modulation is done by the leaves themselves. Patient population and dose prescription Planning objectives were formulated to avoid differences The study enrolled 27 patients with carcinomas in the in the optimization weighting factors and the way these head and neck region. Depending on the location of the are interpreted by the planning software. The primary goal tumor, the PTV included the primary tumor of the oro-, was to treat the PTV with a minimum of 95% and a maxi- hypo-, nasopharynx or larynx and the bilateral cervical mum of 107% of the prescribed dose [18], which should and supraclavicular lymphatic drainage (mean target ideally lead to a median dose of 50 Gy. No dose normali- volume:1209 ± 281 cm ). In the immediate vicinity the zation took place. Furthermore, the volume of the PTV relevant OAR are the spinal cord, the brain stem (depend- which received 90% and 95% of the prescribed dose (V / 90% ing on the tumor location and extent of PTV) and the par- V ) should reach 100% respective 95% of the target 95% otid glands. A total dose of 50 Gy was delivered to the dose. Secondary objectives were the above mentioned PTV including the lymphatic drainage area and boost irra- dose limits to the OAR, provided that the targets goals diation to a maximum of 72 Gy could follow. This might were met satisfactorily. Consistent support structures were result in a wide variation of boost contours (volume, posi- created for all systems in order to steer the optimization tion, prescribed dose), for which a large number of plans and to avoid overdose outside the PTV. These structures with different dose prescriptions would have to be created. will be integrated into the planning process, but no plan- In order to achieve comparability between the systems a ning objectives, in terms of V should be below a certain xGy simple standardization was applied. Plans for the low-dose percentage, were defined for these volumes. Further target area were created and set to a fractionation of 2 Gy attempts were made to achieve the planning specifications in 25 fractions. As the total dose was kept to 50 Gy (and with as few segments as possible. not to 72 Gy as it would be in the curative approach) the maximum dose affecting the spinal cord, parotid glands Evaluation methods and brainstem was reduced proportionally. Thus, the max- The plans were compared and analyzed using DVH. For imum dose to the spinal cord and brainstem was kept all systems, except KonRad, the data are analyzed in each below 30 Gy, and that to the parotid glands below 19 Gy program’s own analysis tool. Since KonRad has no output [16]. Healthy tissue is defined as outer contour of the for specific D -and V -values, the CT scans, dose and x% yGy patient subtracted by the PTV. This volume is limited in volume structures were exported into MasterPlan, where craniocaudal direction. the plans were further analyzed. No recalculation of dose took place. Planning methodology Because of the fact that the dose calculation algo- Thestandardfield setupusedforallplans consistedof rithms of the treatment planning systems (TPS) have seven static 6 MV photon fields with gantry angles of 0°, difficulties and different approaches to model the build- 52°, 104°, 156°, 204°, 256°, 308° with a dose rate of 300 up effect, the PTV was retracted 3 mm from the outline MU/min. All plans were calculated for a clinical used [19].ThismodifiedPTV wasusedfor furtheranalysis. Artiste Linac (Siemens AG, Erlangen, Germany) Maximum doses were included in the assessment by the equipped with a 160 Leaf MLC. Further details and dosi- parameter of V . The homogeneity index (HI =[D - 107% 2% metric characteristics were investigated by Tacke et al. D ]/D ) reflects how steep the dose drop off 98% prescription [17]. The planner tried to keep the number of segments in the PTV is. A smaller HI indicates a more homoge- as low as possible while fulfilling the planning goals. The nous dose distribution. maximum segment number allowed for DSS plans were Maximum doses in serial OAR are reflected on D .The 2% kept to a minimum while reaching the planning objec- dose to the major salivary glands was recorded at the med- tives. In order to minimize the number of segments for ian dose. Low-dose exposure of healthy tissue was KonRad the intensity levels and used median filter were reported as the volume which receives 5 Gy (V )and 5Gy varied. Since the number of segments per beam direction 10 Gy (V ). For all plans the treatment times were 10 Gy is fixed at Panther DAO due to user definitions, the plan- measured from the beginning of the first field until the ner was allowed to apply split beams to provide addi- end of the last segment. The efficiency of the IMRT tional degrees of freedom. This procedure is doubling the method was derived from the calculated MU and required beam from one direction and therefore doubling the segments. number of segments from this particular direction. The To quantify the differences of parameters between two cut off for all systems per segment was set to 5 MU with systems a test of significance is required. Since the mea- a minimum field size of 4 cm . surements were collected for each planning system for Sabatino et al. Radiation Oncology 2012, 7:7 Page 4 of 8 http://www.ro-journal.com/content/7/1/7 the same collective, a two-sided, paired student t-test was and has a low standard deviation (0.01). However, statis- used. Statistical significance was defined for p-values < tical significant differences for the HI were found for all 0.05. A Kolmogorov-Smirnov-test was applied before, planning systems. The p-values are < 0.001 for DSS vs. whereby the parameters were tested with regard to a nor- Panther DAO and KonRad, and Panther DAO vs. mal distribution. KonRad. All results of this study are reported as averages of the entire patient cohort and the appropriate standard Organs at risk and low dose exposure deviation. The results of DVH analysis for the OAR are listed in Table 2. Only KonRad met the planning objectives for Results the serial riskstructures. The average maximum doses to Dose-coverage for PTV the spinal cord are 30.0 Gy (KonRad), 30.6 Gy (DSS) All IMRT systems reach satisfactory and comparable and 31.5 Gy (Panther DAO). KonRad is best in sparing results for the dose in the PTV. The prescribed median the brainstem with 26.8 Gy followed by DSS (27.2 Gy) dose of 50 Gy is achieved in all cases. Figure 1 shows and Panther DAO (30.9 Gy). Statistical significant differ- theaverage DVHfor the entirepatient cohort forthe ences are observed between Panther DAO and the two three TPS. other TPS. The prescribed high-dose objectives for V and V The planning objective for major parotid gland sparing 90% 95% come close to the requested aim (100% respective 95%). was difficult to achieve. The median dose varied The volume which receives more than 107% of the pre- between 19.7 Gy (DSS) to 21.0 Gy (Panther DAO) and scription dose is lowest for DSS (0.3%) followed by 21.3 Gy (KonRad) (see Table 2). Panther DAO (0.7%) and KonRad (1.2%). Table 1 shows The exposure of healthy tissue to doses below 5 Gy and the results for the PTV according to the DVH analysis. 10 Gy is presented in Table 2. Statistical significance are The HI is within a close range for all tested systems observed for all values and planning systems except for (0.12 for DSS; 0.13 for Panther DAO; 0.14 for KonRad) V between KonRad and DSS. The exposure at this 5Gy DAO DSS KonRad 30 40 50 Dose [Gy] Figure 1 Dose volume histogramm for the PTV for the three tested IMRT-systems.Shown is theaverage DVHfor thewhole patient cohort, different colours denoting the different TPS. Volume [%] Sabatino et al. Radiation Oncology 2012, 7:7 Page 5 of 8 http://www.ro-journal.com/content/7/1/7 Table 1 Dosimetric results for the PTV from 2-step and Table 3 Average MU and treatment time for the three direct optimized IMRT for the tested TPS. different optimization systems. Parameter DAO KonRad DSS p Parameter DSS KonRad DAO Median [Gy] 50.3 ± 0.1 50.3 ± 0.1 50.3 ± 0.1 - MU 807 ± 110 564 ± 78 308 ± 21 V [%] 0.7 ± 0.4 1.2 ± 0.6 0.3 ± 0.5 a, b, c reduction - 30% 62% 107% V [%] 94.7 ± 0.8 94.2 ± 1.3 94.7 ± 1.1 a treatment time [min] 10.5 ± 1.2 9.75 ± 1.2 7.0 ± 0.9 95% V [%] 99.1 ± 0.4 98.7 ± 1.1 99.1 ± 0.4 - reduction [min] - 0.75 3.5 90% HI [-] 0.13 ± 0.01 0.14 ± 0.01 0.12 ± 0.01 a, b, c Given are the calculated average MU for a 2 Gy fraction, the average treatment time and the respective reduction to the highest values (DSS). Given is the median dose, the volume which receives 107%, 95% and 90% of the prescribed dose as well as the homogeneity index. Statistical significance (p < 0,05): a: Panther DAO vs KonRad, b: Panther DAO In terms of PTV coverage and OAR sparing all sys- vs DSS, c: KonRad vs DSS, tems reach satisfactory and clinically acceptable results, even though some statistical significant differences can dose level is lowest for Panther DAO followed by Kon- be observed. But the clinical relevance at this level is Rad and DSS. questionable. Similar conclusions could be stated for the risk structures spinal cord, brainstem and parotid Evaluation of efficiency glands. Panther DAO is violating the planning objectives The number of MU for a 2 Gy fraction resulted in 308 ± but if one projects this percentage dose reduction to a 21 MU for Panther DAO, 564 ± 78 MU for KonRad and totalprescriptiondoseof72Gy, sparingofthe spinal 807 ± 101 MU for DSS. Compared to DSS the percen- cord is possible. Greater variations were observed in the tages of MU reduction are 30% (KonRad) and 62% efficiency of the intensity modulation. Compared to the (Panther DAO). The results are shown in table 3 and as 2-step approach the direct optimization algorithms are boxplot diagrams for the obtained MU in Figure 2. In a able to decrease the number of segments. DSS generates similar way the required segments are shown in Figure 3. 22% and Panther DAO 37% less segments than KonRad. On average 43 ± 9 segments are needed for Panther One aim was to keep the number of segments per plan DAO, 53 ± 8 segments for DSS and 68 ± 7 segments for as low as possible while reaching the planning goals for KonRad. PTV coverage and OAR sparing. One reason for the Treatment times were measured for all patients and efficiency of Panther DAO is the possibility to create a planning systems. The longest average treatment time was highly modulated field with few segments. With 5 seg- 10.5 ± 1.2 min for DSS. For the KonRad system 9.75 ± 1.2 ments per beam, 31 intensity levels (intensity levels = min were observed and 7.0 ± 0.9 min for Panther DAO. 2 -1) can be generated [11]. Concerning calculated MU Figure 4 shows the correlation between the MU and for a 2 Gy fraction the purely direct optimization system the number of segments. For DSS, the relationship has the lowest values. The reduction for Panther DAO is between these variables is most pronounced (R = 62% and for KonRad 30% compared to DSS. 0,668). A moderate increase is recorded with the Kon- Statistical differences were found for the low dose Rad (R = 0.335) system. Theoretically 80 segments with exposure. Two reasons could be responsible for this Panther DAO (R = 0.541) would not exceed 400 MU. finding: chosen gantry angles and dose calculation algo- rithms. In this case the large deviations could not be Discussion explainedbythegantryangles sincetheywereall the This study compares step&shoot IMRT for head and same for all plans and systems. neck tumors at standardized conditions with special The presumption is that the differences occur due to attention to their different optimization approaches. less MU. But since the TPS calculations accuracy for Table 2 Dosimetric results for spinal cord, brainstem, summed parotid glands and healthy tissue. Organ Parameter DAO KonRad DSS p Spinal cord D [Gy] 31.5 ± 1.1 30.0 ± 1.6 30.6 ± 1.4 a, b 2% Brainstem D [Gy] 31.0 ± 2.2 26.8 ± 2.6 27.2 ± 2.6 a, b 2% Summed Parotids D [Gy] 21.0 ± 1.8 21.3 ± 2.1 19.7 ± 1.8 b, c Median Healthy Tissue V [%] 66.7 ± 5.4 73.5 ± 5.4 73.6 ± 4.2 a, b 5Gy Healthy Tissue V [%] 53.8 ± 5.8 57.8 ± 5.2 59.4 ± 3.8 a, b, c 10 Gy Given is the dose in Gy to two percent of the volume of the spinal cord and brainstem, median dose to the parotid glands and the volume of healthy tissue receiving more than 5 Gy respective 10 Gy. Statistical significance (p < 0,05); a: Panther DAO vs KonRad, b: Panther DAO vs DSS, c: KonRad vs DSS Sabatino et al. Radiation Oncology 2012, 7:7 Page 6 of 8 http://www.ro-journal.com/content/7/1/7 Figure 2 Boxplot diagram of monitorunit distribution calculated from the three tested IMRT-systems. The median is shown as a blue st rd line, maximum and minimum in red, and 1 and 3 quartile as thin black lines. low doses is reduced, an experimental measurement (R = 0.541) can be expected. Rather a further increase could verify these findings. of segments could result in potentially dosimetric For Panther DAO - even if highly rising numbers of unstable conditions, as the number of MU per segment segments occurred - only a moderate increase of MU may be too small. A stronger correlation was found for Figure 3 Boxplot diagram of required segments for the three tested IMRT-systems. The median is shown as a blue line, maximum and st rd minimum in red, and 1 and 3 quartile as thin black lines. Sabatino et al. Radiation Oncology 2012, 7:7 Page 7 of 8 http://www.ro-journal.com/content/7/1/7 Figure 4 Linear correlation between number of segments and MU. The MU are given according to the number of segments for each TPS. The quality of the correlation is given with R . Lines indicate fitted linear correlation and the corresponding equation is given. DSS (R = 0.669). A larger variation of the pair of values less segments. The exposure time is reduced by 29%. Dobler et al. evaluated the effects of DSS compared to occur in KonRad (R = 0.335). Theprimary focusofthisstudy is the MU efficiency the 2-step approach of MasterPlan [3]. The study was of the compared optimization algorithm. This is owed conducted with 10 patients with a hypopharyngeal carci- to the increasing number of MU in IMRT in compari- noma with the same field arrangement and fraction dose son to 3D-conformal radio therapy (3DCRT) which as in this study. A reduction in MU from 1151 to 901 could increase the risk of radiation induced secondary was found in favor of the direct optimization procedure. malignancies due to scattered radiation. Panther DAO The required average segment number of 77 is the same plans could decrease the amount of scatter radiation ori- for both approaches. In a further planning study con- ginating from the collimator head. Hall pointed out the cerning head and neck tumors with integrated boost, need for protection of patients from scattered radiation Wiezorek et al. compared static and rotational IMRT in IMRT-treatments [12]. He reported a potential and Tomotherapy as well as different optimization algo- increase of radiation-induced cancer due to larger total- rithms [9]. Normalized MU were found to be lowest for body doses caused by leakage radiation. Considering this Panther DAO. aspect, MU reduced plans with comparable quality The low number of MU and segments is the main should be preferred, especially for pediatric cases or dis- reason for the shortest treatment times for Panther eases of young adults and adolescents with highly cur- DAO. On average, the amount of time saved is 3.5 min able concepts. (DSS) and 2.8 min (KonRad). These time savings could The reported reductions are in agreement with pub- be used for image guidance. In addition to that it is lished studies. Jones et al. compared 2-step IMRT with advantageous for intrafractional movement of the organs directly optimized IMRT plans using the Pinnacle and for the comfort of the patient. A reason for these DMPO in a planning study for head and neck tumors time savings is the above mentioned creation of fluence [5]. For this system an approach for the direct optimiza- modulation. Within a field the modulation is done by the fast leaves while the slower jaws are fixed to one tion, which is similar with DSS, is implemented. It is reported that DMPO requires 42% less MU and 35% position. Sabatino et al. Radiation Oncology 2012, 7:7 Page 8 of 8 http://www.ro-journal.com/content/7/1/7 sequencing in IMRT of hypopharyngeal carcinoma. Radiat Oncol 2007, In the planning study by Wiezoreck et. al the exposure 2:33. of healthy tissue to low doses were also evaluated. The 4. Fogliata A, Bolsi A, Cozzi L: Comparative analysis of intensity modulation low doses to healthy tissue were found to be highest for inverse planning modules of three commercial treatment planning systems applied to head and neck tumour model. Radiother Oncol 2003, the Panther DAO system [9]. These findings differ from 66:29-40. the results of our study. Differences may occur due to 5. Jones S, Williams M: Clinical evaluation of direct aperture optimization different approaches to calculate the values for the low when applied to head-and-neck IMRT. Med Dosim 2008, 33:86-92. 6. Reitz B, Miften M: Comparison of the KonRad IMRT and XiO treatment dose exposure. In this study the external was subtracted planning systems. J Appl Clin Med Phys 2008, 9:2770. by the PTV and the extent of this new volume limited 7. Vanetti E, Clivio A, Nicolini G, et al: Volumetric modulated arc to 3 cm in craniocaudal direction from the PTV. This radiotherapy for carcinomas of the oro-pharynx, hypo-pharynx and larynx: a treatment planning comparison with fixed field IMRT. Radiother was done because of a limited calculation matrix in Oncol 2009, 92:111-117. KonRad. 8. Verbakel WF, Cuijpers JP, Hoffmans D, Bieker M, Slotman BJ, Senan S: Another reason could be the number of chosen gantry Volumetric intensity-modulated arc therapy vs. conventional IMRT in head-and-neck cancer: a comparative planning and dosimetric study. Int angles. In the study of Wiezoreck et al. eleven beam J Radiat Oncol Biol Phys 2009, 74:252-259. directions were taken for Panther DAO. 9. Wiezorek T, Brachwitz T, Georg D, et al: Rotational IMRT techniques compared to fixed gantry IMRT and tomotherapy: multi-institutional planning study for head-and-neck cases. Radiat Oncol 6:20. Conclusions 10. Broderick M, Leech M, Coffey M: Direct aperture optimization as a means All IMRT systems are able to calculate acceptable plans of reducing the complexity of Intensity Modulated Radiation Therapy in terms of PTV coverage and OAR sparing. Main differ- plans. Radiat Oncol 2009, 4:8. 11. Shepard DM, Earl MA, Li XA, Naqvi S, Yu C: Direct aperture optimization: a ences are observed in the efficiency of the fluence modu- turnkey solution for step-and-shoot IMRT. Med Phys 2002, 29:1007-1018. lation. Based on the results of published literature and 12. Hall EJ: Intensity-modulated radiation therapy, protons, and the risk of the results of this study, a further reduction of plan com- second cancers. Int J Radiat Oncol Biol Phys 2006, 65:1-7. 13. Webb S: The physical basis of IMRT and inverse planning. Br J Radiol plexity can be stated for the purely direct-optimizing 2003, 76:678-689. Panther DAO system in IMRT planning of complex head 14. Oelfke U, Nill S, Wilkens JJ: Physical Optimization.Edited by: Bortfeld TR, and neck cases. The reduced number of segments and Schmidt-Ullrich R, De Neve W, Wazer DE. Image-guided IMRT, Berlin Heidelberg: Springer; 2006:31-46. MU should lead to less leakage radiation from the colli- 15. Hardemark B, Liander A, Rehbinder H, Löf J: Direct machine parameter mator head. If and how much these reductions lead to optimization with RayMachine in Pinnacle. Ray-Search White Paper 2003. less peripheral doses should be verified by experimental 16. Marks LB, Yorke ED, Jackson A, et al: Use of normal tissue complication probability models in the clinic. Int J Radiat Oncol Biol Phys 76:S10-19. measurements as performed by Wiezorek et al. [20]. 17. Tacke MB, Nill S, Haring P, Oelfke U: 6 MV dosimetric characterization of the 160 MLC, the new Siemens multileaf collimator. Med Phys 2008, 35:1634-1642. Author details 18. ICRU Report 50-Prescribing, recording and reporting photon beam Department of Radiation Therapy and Radiooncology, Radiologische Allianz therapy. International Commission on Radiation Units and Measurements Hamburg, Hamburg, Germany. Institute of Radiation Protection and Medical Physics,Technische Hochschule Mittelhessen, Giessen, Germany. 19. Chung H, Jin H, Dempsey JF, et al: Evaluation of surface and build-up region dose for intensity-modulated radiation therapy in head and neck Authors’ contributions cancer. Med Phys 2005, 32:2682-2689. MS and MK contributed significantly to study design and concept. MS was 20. Wiezorek T, Schwahofer A, Schubert K: The influence of different IMRT responsible for treatment planning. Data analysis and interpretation of techniques on the peripheral dose: a comparison between sMLM-IMRT results were performed by MS and MK. Corrections and/or improvements and helical tomotherapy. Strahlenther Onkol 2009, 185:696-702. were suggested by MK, KZ and FW. FW was responsible for clinical evaluation of the treatment plans. All authors read and approved the final doi:10.1186/1748-717X-7-7 manuscript. Cite this article as: Sabatino et al.: The impact of direct aperture optimization on plan quality and efficiency in complex head and neck Competing interests IMRT. Radiation Oncology 2012 7:7. The Department of Radiation Therapy and Radiooncology of the Radiologische Allianz Hamburg receives research grants from Prowess Inc., Siemens AG Healthcare sector and Nucletron BV. Received: 30 August 2011 Accepted: 23 January 2012 Submit your next manuscript to BioMed Central Published: 23 January 2012 and take full advantage of: References • Convenient online submission 1. Bertelsen A, Hansen CR, Johansen J, Brink C: Single Arc Volumetric Modulated Arc Therapy of head and neck cancer. Radiother Oncol 2010, • Thorough peer review 95:142-148. • No space constraints or color figure charges 2. Cozzi L, Fogliata A, Bolsi A, Nicolini G, Bernier J: Three-dimensional • Immediate publication on acceptance conformal vs. intensity-modulated radiotherapy in head-and-neck cancer patients: comparative analysis of dosimetric and technical parameters. • Inclusion in PubMed, CAS, Scopus and Google Scholar Int J Radiat Oncol Biol Phys 2004, 58:617-624. • Research which is freely available for redistribution 3. Dobler B, Pohl F, Bogner L, Koelbl O: Comparison of direct machine parameter optimization versus fluence optimization with sequential Submit your manuscript at www.biomedcentral.com/submit

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Radiation OncologySpringer Journals

Published: Jan 23, 2012

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