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Background: A study was realised to evaluate and determine relative figures of merit of a new algorithm for photon dose calculation when applied to inhomogeneous media. Methods: The new Acuros XB algorithm implemented in the Varian Eclipse treatment planning system was compared against a Monte Carlo method (VMC++), and the Analytical Anisotropic Algorithm (AAA). The study was carried out in virtual phantoms characterized by simple geometrical structures. An insert of different material and density was included in a phantom built of skeletal-muscle and HU = 0 (setting “A”): Normal Lung (lung, 0.198 g/cm ); Light Lung 3 3 (lung, 0.035 g/cm ); Bone (bone, 1.798 g/cm ); another phantom (setting “B”) was built of adipose material and 3 3 3 3 including thin layers of bone (1.85 g/cm ), adipose (0.92 g/cm ), cartilage (1.4745 g/cm ), air (0.0012 g/cm ). 2 2 Investigations were performed for 6 and 15 MV photon beams, and for a large (13 × 13 cm ) and a small (2.8 × 13 cm ) field. Results: Results are provided in terms of depth dose curves, transverse profiles and Gamma analysis (3 mm/3% and 2 mm/2% distance to agreement/dose difference criteria) in planes parallel to the beam central axis; Monte Carlo simulations were assumed as reference. Acuros XB gave an average gamma agreement, with a 3 mm/3% criteria, of 100%, 86% and 100% for Normal Lung, Light Lung and Bone settings, respectively, and dose to medium calculations. The same figures were 86%, 11% and 100% for AAA, where only dose rescaled to water calculations are possible. Conclusions: In conclusion, Acuros XB algorithm provides a valid and accurate alternative to Monte Carlo calculations for heterogeneity management. Keywords: dose calculation algorithm, Acuros, AAA, VMC++, inhomogeneity Background et al [1]), according to management (type b) or non A new photon dose calculation algorithm has recently management (type a) of the electron transport in dose been implemented in the Eclipse treatment planning calculation. “Type b” algorithms present higher accuracy system (Varian Medical Systems, Palo Alto, USA). This in heterogeneous media, in particular for very low den- algorithm, named Acuros XB Advanced Dose Calcula- sity tissues [2]. The differences observed in phantom tion (Acuros XB in the following) belongs to the class of studies are partially mitigated in patients, where there is the Linear Boltzmann Transport Equation (LBTE) Sol- a predominance of soft tissues, more similar to water vers. LBTE solvers, similarly to those used in Monte [1]; in cases with large volumes of air or low density Carlo methods, aim to allow for accurate modelling of media the differences remained largely in favour of dose deposition in media. “type b” models [3]. Many studies explored the accuracy of algorithms for Many studies have also been published to compare dif- photon dose calculation in materials different from ferent algorithms with Monte Carlo simulations or mea- water. In 2006 a classification was proposed dividing surements: the Anisotropic Analytical Algorithm (AAA) algorithms into “type a” and “type b” groups (Knöös was evaluated e.g. by van Esch [4], Fogliata [2], daRosa [5]. Results showed that accuracy significantly depends on energy, field size, and density of the materials. Algorithms * Correspondence: afc@iosi.ch allowing calculation of dose-to-medium lead to better Medical Physics Unit, Oncology Institute of Southern Switzerland, Bellinzona, Switzerland © 2011 Fogliata 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. Fogliata et al. Radiation Oncology 2011, 6:82 Page 2 of 15 http://www.ro-journal.com/content/6/1/82 agreement with Monte Carlo as already shown by Siebers In brief, the dose D in any grid voxel i is given by the [6] and confirmed by Knöös [1]. The clinical applicability following equation [13]: of dose-to-medium calculations is limited to few systems and the new Acuros XB is included in this list. The first σ (r, E) ED e ˆ ˆ D = dE d (r, E, ) works on validation and evaluation of the Acuros XB algo- ρ(r) 0 4π rithm were recently published by Fogliata et al [7] and Bush et al [8] showing very promising results compared to where s is the macroscopic electron energy deposi- ED both measurements and Monte Carlo calculations. tion cross section, r the material density, and Ψ the The present report summarises a study conducted to angular electron fluence. Acuros XB calculates the investigate the performance and accuracy of the Acuros energy dependent electron fluence, based on the mate- XB in its Eclipse implementation, when applied to mate- rial properties of the patient, as derived from the rials different from water. Tests are performed in simple Hounsfield Unit (HU) of the CT dataset. geometrical phantoms with inserts or layers of different Dose to medium or dose to water can be selected in materials for photon beams. Acuros XB calculations are Acuros XB. performed using both dose-to-medium and dose-to- When dose to medium is calculated, s and r and ED water options. The validation assumes, as benchmark, are based on the material properties of output grid Voxel Monte Carlo (VMC++) simulations. To complete voxel, i. the comparative analysis, results are reported also for When dose to water is reported, s and r are based ED the latest version of the AAA, the “type b” algorithm on water in a post processing step (the transport calcu- currently implemented in the Eclipse TPS. lation is identical for both dose to medium and dose to water reporting); in materials different from water, the Methods dose is defined as the dose absorbed by a volume of The algorithms water which is small enough not to perturb the energy The Acuros XB Advanced Dose Calculation algorithm dependent electron fluence. This volume should be The Acuros XB is based on the application of the LBTE much smaller than the output dose grid voxel of the that describes the interactions of radiation particles with computer based calculation or of any detector used to matter. This is based on approximate numerical meth- measure dose to water. ods. Monte Carlo (MC) and explicit LBTE solvers, as The macroscopic cross sections s used by Acuros XB Acuros XB, should converge to the same final results. In are modelled as the product of two components: the practice, both methods are affected by potential inac- microscopic cross section for a given interaction ,and σ ˜ curacies depending on the level of sampling of the prob- the mass density of the material r with the relationship: ability distribution functions applied during MC simulations or to the application of variables discretisa- N ρ σ = σ ˜ tion during explicit LBTE solution. A characteristic of LBTE solvers, compared to MC simulations, is the absence of uncertainties due to statistical noise in the where M is the atomic mass and N is the Avogadro’s calculated dose. number. Coupled photon-electron cross sections include Progenitor of Acuros is the Attila algorithm [9], devel- Compton scatter, photo-electric effect, and pair produc- oped originally for nuclear physics applications, and also tion, but not the Rayleigh scatter. In the model, the investigated for external photon beam dose calculations energy from bremsstrahlung photons produced by elec- [10,11] and brachytherapy [12]. The new Acuros algo- tron interactions inside the patients is not considered, rithm, based on many of the Attila methods, was adapted being judged not significant for energies typical in the for external photon dose calculations and described in radiotherapy range. Vassiliev et al [13]. Acuros XB is the Varian implementa- The cutoff for electron energy is set at 500 keV (200 tion in the Eclipse planning system of the original Acuros keV in version 11) kinetic energy only (without rest algorithm. mass) and it is not modifiable by the user. Acuros XB implementation consists of two main com- In clinical cases, radiation transport is performed for materials derived by anatomical information: tissue seg- ponents: i) the photon beam source model and ii) the mentation is based on density ranges related to HU values radiation transport model. read in the patient CT dataset. Density to human tissues The latter includes discretisation of the spatial ( ), correspondence is reported in Table 1 (for both Acuros energy (E), and angular ( ˆ ) variables and was firstly XB versions 10 and 11); for each material the specific che- described by Vassiliev et al [13] and summarised in a mical elemental composition is based on the ICRP Report previous report on Acuros XB validation in water for 23 [15]. In Eclipse the user can also manually assign simple fields [14]. Fogliata et al. Radiation Oncology 2011, 6:82 Page 3 of 15 http://www.ro-journal.com/content/6/1/82 Table 1 Material mass densities for automatic conversion, for all algorithms used: Acuros XB, AAA and VMC++. as implemented in the two Acuros XB versions For a detailed description the reader can refer to Tilli- 3 3 Material Density Range [g/cm ] Density Range [g/cm ] kainen et al [19]. Acuros XB version 10 Acuros XB version 11 Air - 0.000-0.020 Eclipse framework and tested versions Lung 0.000-0.590 0.011-0.624 All calculations are performed using the Eclipse plan- Adipose Tissue 0.590-0.985 0.554-1.001 ning system, with version 10 for Acuros XB and AAA, Muscle, Skeletal 0.985-1.075 0.969-1.093 and version 8 for VMC++. The algorithm versions used Cartilage 1.075-1.475 1.056-1.600 are as following: Bone 1.475-3.000 1.100-3.000 � Acuros XB: clinical release 10.0.28. � AAA: clinical release 10.0.25. � VMC++: research release 8.0.1, not for clinical usage. Some results are also reported for the Acuros XB cal- materials (or human tissues) with predefined HU values. culations in its engineering pre-clinical version 11.0.03. Acuros XB does not allow calculations for mass densities Two are the main differences between the two Acuros higher than 3.0 g/cm to prevent incorrect material assign- XB versions 10 and 11. The first concerns the human ment to densities larger than the expected scale for human material assignment, where the Air material is assigned bones. If the CT dataset contains HU values higher than to very low density regions in the body (Air material is this upper limit, ad-hoc structures are defined with man- not present in version 10), and the density ranges for ual assignment of materials. each material are slightly overlapping (Table 1). The The Anisotropic Analytical Algorithm, AAA Comparison to the AAA algorithms was also included in second improvement refers to a better re-sampling pro- the study. AAA, based on the work of Ulmer et al cess of the structure voxels to the calculation grid, set- [16-18] and Tillikainen et al [19,20], was extensively ting the density and material of the structure to the validated [2,4,21-26]. The reader should refer to Tillikai- calculation voxel when at least half of the calculation nen et al [20] for detailed description. AAA is not voxel volume belongs to the structure. accounting for chemical material/tissue properties, All calculations, are performed with a grid size of 1.25 hence the computed dose can be defined as dose to mm. The grid, in addition to the smoothing process water, rescaled according to the specific density (dose used in the VMC++ calculations might lead to some rescaled to water in the following). unavoidable smoother dose profiles. The Voxel Monte Carlo, VMC++ The Voxel Monte Carlo VMC++ [27-30] is a class II The phantoms and the beams All studies are performed on a set of virtual phantoms. condensed history Monte Carlo simulation of coupled Figure 1 shows a schematic representation of the electron-photon transport. It uses small angle approxi- phantoms which are characterized as follows [2]: mation, and re-uses electron histories and STOPS Phantom A) An insert, covering laterally only half of (Simultaneous Transport Of Particle Sets) variance the entire phantom and positioned at 5 cm depth in a reduction techniques [31]. It was validated in the field large phantom of HU = 0 (’Muscle Skeletal’ as auto- of radiotherapy by Gardner et al [32]. matic assignment) is simulated for three different mate- The version of VMC++ used here is implemented as a rials and thicknesses: research version in Eclipse. Material chemical composition and related density ranges are here set identical to the - Normal Lung: 0.198 g/cm , HU = -780, lung tissue, Acuros XB settings. For the simulations, the electron 16 cm thick. energy cutoff is automatically selected and based upon the density of the material density; a smoothing process is acti- - Light Lung: 0.035 g/cm , HU = -942, lung tissue, vated during calculations (locally adaptive Savitzky-Golay 16 cm thick. filter); final dose calculation accuracy is set to 1%. A cross - Bone: 1.798 g/cm , HU = 1380, bone tissue, 6 cm validation of VMC++ version is here presented against thick. EGSnrc as already published [2]. During EGSnrc simula- tions 75 million particles are used to have a maximum sta- Phantom B) Four thin layers of different materials tistical uncertainty of about 2%. The resolution is 2.5 mm (Heter1), starting at 5 cm depth are included in a large in all directions. The total energy cut-off for electrons and phantom of the same density and composition as insert photons are set to 700 keV and 10 keV, respectively. C (Adipose): Source model The source model used for this study is the standard - Layer A: 1.4751 g/cm , HU = 763, bone tissue, multiple source implemented in Eclipse and is the same 1 cm thick D[LVD[LV LQBLQB Fogliata et al. Radiation Oncology 2011, 6:82 Page 4 of 15 http://www.ro-journal.com/content/6/1/82 SSD=87cm Phantom A, 2-D analysis Phantom A Phantom B 5cm 5cm HH HH A JJ GG D HH BB QQ LL BB WW XX RR WW HU=0 (Adipose) Material as C A=16cm for Lung A=1cm (Bone) B=1.6cm (Air) A=6cm for Bone C=2cm (Adipose) D=1cm (Cartilage) Figure 1 Geometrical layout of the phantoms. Phantom A on the left; phantom B in the middle; sectors used in the 2D gamma analysis for phantom A on the right. - Layer B: 0.0012 g/cm , HU = -993, air, 1.6 cm The analysis thick 1-D analysis: DD and profiles - Layer C: 0.92 g/cm , HU = -122, adipose tissue, Data are reported for calculations along the directions 2 cm thick shown by the arrows in figure 1, i.e. depth dose curves - Layer D: 1.4745 g/cm , HU = 762, cartilage tissue, (DD) at -4 cm off-axis parallel to the beam central axis 1 cm thick. for phantom A, and on the beam central axis for phan- tom B. Notice that layers A and D differ for only one HU, but Horizontal transverse profiles are calculated at the have different material assignment (bone or cartilage), depth of mid-thickness of the inhomogeneities for phan- presenting different elemental composition, especially in tom A to evaluate the lateral interface. terms of Calcium content. 2-D analysis: Gamma evaluation Source to phantom distance SSD is set to 87 cm, gan- 2-D dose distributions in the vertical transversal plane try and collimator to 0 degree. Doses are normalised to through the isocentre, crossing the longest field jaw set- 3 cm depth on the beam central axis. For all phantoms ting are evaluated. Gamma of Low analysis [33] is per- calculations are performed for the following settings: formed, using different threshold criteria: distance to agreement DTA = 2 mm and 3 mm, dose difference ΔD = 2%, 3%; all calculations are performed as global - field sizes: 2.8 × 13 cm , small field, SF, (the long gamma indexes, i.e. relative to the dose at 3 cm depth axis crossed the heterogeneity boundary), and 13 × on the beam central axis. VMC++ calculations are 13 cm , large field, LF. assumed as reference. Each planar dose from phantom - beam energies: 6 and 15 MV from a Varian Clinac A is divided into various sectors as depicted in figure 1: 2100 iX, presenting TPR of 0.672 and 0.761 20/10 respectively (6X and 15X in the following). - pre: before the inhomogeneity, from 3 cm depth, For all cases, calculations are performed for Acuros with 1.5 cm internal margin from the field edge on XB and VMC++ as: i) dose to water, ii) dose to medium the left and the beam central axis on the right and iii) dose rescaled to water. This last modality is - in: inside the inhomogeneity, with the same lateral defined with a manual assignment to water material for margins of 1.5 cm all phantom structures, outline and inserts, with specific - post: after the inhomogeneity for a depth of 2 cm. HU according to each phantom setting; CT ranges to - edge: along the inhomogeneity, across the field corresponding materials and compositions are modified edge, 1.5 cm inside and 1.5 cm outside the border accordingly also for VMC++ calculations. For AAA, - edge_in: the edge sector only inside the field only the dose rescaled to water option is available. - edge_out: the edge sector only outside the field out out_ _ SRVSRV LQLQ SUSU Fogliata et al. Radiation Oncology 2011, 6:82 Page 5 of 15 http://www.ro-journal.com/content/6/1/82 - axis: across the beam central axis (and also inho- VMC++ vs. EGSnrc comparison mogeneity), 1.5 cm inside and 1.5 cm outside the Due to the non-validated nature of the used VMC++, in inhomogeneity figure 3 a comparison between VMC++ and EGSnrc - axis_in:the axis sector only inside the simulations for DD curves in SF and 6X cases of phan- inhomogeneity tom A is presented, showing small differences between the two calculations. - axis_out:the axis sector only outside the inhomogeneity One-dimensional analysis: DD and profiles In the following, only graphs relative to dose to medium Gamma evaluation is recorded as Gamma Agreement Index, GAI, defined as the percentage of the pixels ful- calculations for Acuros XB, AAA and VMC++ are pre- filling the criteria inside each sector. sented. Graphs referring to dose to water and dose For phantom B the following regions, included in the rescaled to water are reported as additional files. field, are analysed: For phantom A figure 4 reports the DD curves, while figure 5 shows the horizontal profiles at mid-depth of - pre: before the first inhomogeneity, from 3 to 5 cm the insert. Figure 6 presents the DD curves for phantom depth B. The corresponding additional figures are: Additional - bone: inside the bone layer of 1 cm file 1, Figure S1 (DD, dose to water), Additional file 2, - air: inside the air layer of 1.6 cm Figure S2 (DD, dose rescaled to water), Additional file 3, - adipose: inside the adipose layer of 2 cm Figure S3 (profiles, dose to water), Additional file 4, - cartilage: inside the cartilage layer of 1 cm Figure S4 (profiles, dose rescaled to water), Additional - post: after the last inhomogeneity layer, for 2 cm file 5, Figure S5 (DD, dose to water in phantom B), depth. Additional file 6, Figure S6 (DD, dose rescaled to water in phantom B). To appraise the improvement of Acuros XB compared to previous analytical algorithms, the Results and Discussion AAA calculations are always reported although Acuros Dose to medium, dose to water, dose rescaled to water XB computes transport and dose deposition in the actual material, while in AAA the transport and dose A summary of the DD calculated with Acuros XB as deposition uses radiological and density scaling meth- dose to medium, dose to water, and dose rescaled to ods. A genuine comparison for AAA calculations is pro- water is reported in figure 2(A) for all phantom A, and vided in figures ADD-b referring to dose rescaled to in figure 2(B) for all phantom B settings. Similar results water, where explicit different elemental composition of are found for VMC++ with the three calculation modalities. materials is not considered, and the differences mainly The Lung cases present very small differences among due to the algorithms as radiation transport models are all calculation modalities. In the Bone case, dose to shown. water calculations in bone show strong difference in DD For all calculations performed in Normal Lung tissue, compared to the other two calculations. Dose to med- good agreement between Acuros XB and VMC++ is ium is expressed as dose to water multiplied by the achieved. AAA, as expected from the radiation transport stopping power ratio s between the two model, is less accurate especially for small fields and water,medium media; s is in the range 1.09-1.15 for cortical high energy beams [2]. The rebuildup curve behind the water,bone bone [6]. This confirms the difference of ~10-14% low density insert starts at the interface layer in Acuros reported here. From a qualitative analysis of the Bone XB calculations, while in VMC++ computations it starts DD, a small peak about 2-3 mm before and behind the about 1 mm inside the lung insert. This effect, more evi- insert is computed in the dose to medium with Acuros dent for the Light Lung cases, yields to a shift of about XB. Small peaks in the dose to medium calculations are 2 mm of the rebuildup portion of the curve for the two consistently present also in the horizontal profiles at the algorithms. This difference could partly ascribed to the level of the interface between the two media. boundary handling from different algorithms (consider- Phantom B data show similar patterns depending on ing that no grid alignment is performed between image the layer material, with enhanced criticalities due to the and dose grid voxels), or also to the variance reduction techniques implemented in VMC++ to decrease statisti- short distance between interfaces and to the presence of cal noise. different adjacent materials with very different density The Light Lung DD curve has a noticeably steeper and composition, e.g. bone and air, where the different gradient that starts 2-4 cm distal to the interface. The exit dose from bone is reflected in higher dose inaccu- horizontal profiles through the light lung insert enhance racy in the next air layer. Fogliata et al. Radiation Oncology 2011, 6:82 Page 6 of 15 http://www.ro-journal.com/content/6/1/82 A) B) Figure 2 Depth dose curves (DD) as dose to medium, dose to water, dose rescaled to water. Calculations with Acuros XB version 10 algorithm: (A) Phantom A: in columns: Normal Lung, Light Lung, Bone; in rows: SF and LF for 6X, SF and LF for 15X. (B) Phantom B: in columns: SF, LF; in rows: 6X, 15X. Fogliata et al. Radiation Oncology 2011, 6:82 Page 7 of 15 http://www.ro-journal.com/content/6/1/82 Figure 3 EGSnrc and VMC++ comparison. Depth dose curves (DD) at -4 cm off-axis for the SF, 6X case in Normal Lung, Light Lung and Bone for EGSnrc and VMC++. Figure 4 Depth dose curves (DD) at -4 cm off-axis. Dose to medium calculations for VMC++, Acuros XB version 10, and AAA in phantom A. In columns: Normal Lung, Light Lung, Bone; in rows: SF and LF for 6X, SF and LF for 15X. Fogliata et al. Radiation Oncology 2011, 6:82 Page 8 of 15 http://www.ro-journal.com/content/6/1/82 Figure 5 Profiles at mid-depth of the heterogeneity insert. Dose to medium calculations for VMC++, Acuros XB version 10, and AAA in phantom A. In columns: Normal Lung, Light Lung, Bone; in rows: SF and LF for 6X, SF and LF for 15X. the display of this unexpected increase in dose a few cm since this density range enhanced the inaccuracies com- from the field edge and the interface, an effect that is ing from different approximations,as e.g.the energy more pronounced at deeper distances. Inside the most cutoff for electron interactions, present also in Monte internal light lung material the differences between Carlo simulations. Acuros XB and VMC++ are small. The calculations for Acuros XB and VMC++ show good mutual agreement very low densities prove to be critical for all algorithms in the bone tissue, while AAA presents inferior accuracy Fogliata et al. Radiation Oncology 2011, 6:82 Page 9 of 15 http://www.ro-journal.com/content/6/1/82 Figure 6 Depth dose curves (DD) at beam central axis. Dose to medium calculations for VMC++, Acuros XB version 10, and AAA in phantom B. In columns: SF, LF; in rows: 6X, 15X. for low energy. The small peaks, few mm before and 11 inside the Air material layer presents much better after the bone insert, are more pronounced for Acuros agreement with VMC++ calculations, due to the inclu- XB calculations. In dose to water calculations, Acuros sion in the human tissues list of the air material, that XB shows the start of the increase of the depth dose was considered as lung composition in version 10. curve for dose to water ~5 mm before the bone inter- face, while VMC++ anticipates this to ~10 mm before it. Two-dimensional analysis: Gamma evaluation Results from phantom B present the same, but The main limit of any 2D analysis based on Gamma enhanced, patterns and characteristics as phantom A. evaluation is its threshold effect, hence results have to To note is the inability of AAA to properly model the be considered together with the dose profiles shown in presence of thin inhomogeneities. the previous figures. Examples of the pass/fail patterns In figures 7, 8 and 9 the same plots as in figures 4, 5 in the 2D planes analysed with Gamma evaluation, are and 6 show the calculation difference between the two shown in figure 10 for Acuros XB, dose to medium cal- Acuros XB versions, benchmarked to VMC++. With the culations, with a global gamma criteria of 2%, 2 mm. engineering pre-clinical version 11, the rebuildup after Two-dimensional analysis is here reported only for ver- the lung insert, and the interfaces in the horizontal pro- sion 10 of Acuros XB, being the clinical released version files are more accurately modelled due to the better re- at the present stage. sampling of the structure voxel to the calculation voxel. Figure 11 shows the summary of GAI (for global Also the unexpected dose patterns inside light lung gamma calculation) for each phantom sector, both insert visible for Acuros XB version 10 tend to disappear field sizes, both energies. Each bin represents the two if version 11 is used. threshold results of 2%, 2 mm (thin cross-hatching) For phantom B settings, that present thin inhomo- and 3%, 3 mm (thick cross-hatching). For each sector geneity layers, a plain improvement is shown with Acuros XB (version 10), as well as AAA were analysed Acuros XB version 11, due to the improved alignment against VMC++ calculations. The Additional file 7, of structures and dose voxels. In addition it can be Figure S7 and the Additional File 8, Figure S8 show all noticed that the dose computed with Acuros XB version cases for dose to water and dose rescaled to water, Fogliata et al. Radiation Oncology 2011, 6:82 Page 10 of 15 http://www.ro-journal.com/content/6/1/82 Figure 7 Depth dose curves (DD) at -4 cm off-axis. Dose to medium calculations for VMC++, Acuros XB versions 10 and 11 in phantom A. In columns: Normal Lung, Light Lung, Bone; in rows: SF and LF for 6X, SF and LF for 15X. respectively. Table 2 summarises values of Gamma Summarising the results from phantom A: the GAI (3%, Agreement Index for 3%, 3 mm thresholds in all calcu- 3 mm criteria) for Acuros XB (version 10), dose to lation modalities for phantom A setting. Data are rela- medium, are in average 100%, 86%, 100%, for Normal tive to the entire insert area crossed by the beam in Lung, Light Lung and Bone cases respectively. The the plane parallel to its central axis and passing same figures are 87%, 19%, 76% for AAA calculations. through the isocentre. Considering the dose rescaled to water, where the Fogliata et al. Radiation Oncology 2011, 6:82 Page 11 of 15 http://www.ro-journal.com/content/6/1/82 Figure 8 Profiles at mid-depth of the heterogeneity insert. Dose to medium calculations for VMC++, Acuros XB versions 10 and 11 in phantom A. In columns: Normal Lung, Light Lung, Bone; in rows: SF and LF for 6X, SF and LF for 15X. comparison with AAA comparison is more relevant, itself, while the crucial point for bone tissue is more GAI results are: 99%, 83%, 100% for Acuros XB, and related to the elemental composition and the ability to 86%, 11%, 100% for AAA. consider it in calculations. Those data imply that for low density materials, more From phantom B results the dose inside the Air layer than the specific modality to compute dose, the critical presents rather low gamma values for both AAA and variable is identified in the mass density of the medium Acuros XB version 10. Fogliata et al. Radiation Oncology 2011, 6:82 Page 12 of 15 http://www.ro-journal.com/content/6/1/82 Figure 9 Depth dose curves (DD) at beam central axis. Dose to medium calculations for VMC++, Acuros XB versions 10 and 11 in phantom B. In columns: SF, LF; in rows: 6X, 15X. The results here presented are in good agreement with In their work, Bush et al, used different elemental what has been published by Bush et al [8] comparing compositions and density ranges for HU to mass density Acuros XB calculations with BEAMnrc/DOSXYZnrc conversion with respect to what is implemented in Monte Carlo simulations. The key point from the two Acuros XB. This discrepancy is not used in the present studies remains the high level of accuracy of Acuros XB paper, where the same Acuros XB chemical composition implementation in Eclipse when simple heterogeneities and density range are set for VMC++ calculations. in phantom are involved. Anyway, different settings Also the electron energy cutoff is different for all cal- have been used in the two studies, mainly in the two culations: 700 keV as kinetic+electron rest mass in Monte Carlo algorithms. Monte Carlo calculation from Bush et al;inVMC++ of NormalLung LightLung Heter1 Bone Figure 10 Gamma maps.Examplesfor LF,15X, dose to medium, Acuros XB version 10 vs. VMC++. Thresholds 2 mm, 2% as global gamma computations. White lines represent the heterogeneity interfaces. Fogliata et al. Radiation Oncology 2011, 6:82 Page 13 of 15 http://www.ro-journal.com/content/6/1/82 LightLung, SF, 6X NormalLung, SF, 6X Bone, SF, 6X Heter1, SF, 6X 100 100 100 100 80 80 80 60 60 40 40 20 20 0 0 AAA Acuros10 AAA Acuros10 AAA Acuros10 AAA Acuros10 NormalLung, LF, 6X LightLung, LF, 6X Bone, LF, 6X Heter1, LF, 6X 100 100 100 100 80 80 80 80 60 60 60 60 40 40 40 20 20 20 0 0 0 AAA NormalLung Acu , S ros F X , 1 B 5X AAA LightLunAc g, S urF o, 1 s15 0X AAA Bone, S Ac F u, 1 ro5 s1 X0 AAA Heter1Ac , Su Fr, 1 os5 1X 0 100 100 100 100 80 80 80 80 60 60 60 40 40 40 20 20 20 0 0 0 AAA Acuros10 AAA Acuros10 AAA Acuros10 AAA Acuros10 NormalLung, LF, 15X LightLung, LF, 15X Bone, LF, 15X Heter1, LF, 15X 100 100 100 100 80 80 80 80 60 60 60 40 40 40 20 20 0 0 AAA Acuros10 AAA Acuros10 AAA Acuros10 AAA Acuros10 Figure 11 Histograms of the GAI. Global gamma calculation for each sector of phantoms A and B, for dose to medium calculations for Acuros XB version 10 (red horizontal hatching) and AAA (green diagonal hatching). Each bin represents the two threshold results of 2%, 2 mm (thin cross-hatching) and 3%, 3 mm (thick cross-hatching). In columns: Normal Lung, Light Lung, Bone, phantom B; in rows: SF and LF for 6X, SF and LF for 15X. the present study it is automatically selected and based its clinical version 10. The comparison is extended upon the density of the material density; it is set to 500 also to the widely used AAA algorithm. Good agree- keV (version 10) or 200 keV (version 11) as kinetic ment between Acuros XB and Monte Carlo is shown, energy only for Acuros XB calculations. even in extreme cases of materials of very low density Those two examples of differences point to unavoidable and for low energy and small fields. Some differences approximations of all dose calculations, including Monte between different algorithms are pointed out at inter- Carlo. Those examples enforce the need of publishing dif- faces between different materials. In those cases, ferent comparisons, presenting various characteristics, in Acuros XB and VMC++ present differences mainly in order to give to the community the opportunity to read the rebuildup region. The agreement in this region about results coming from different approaches. improves with the newer version11ofthe Acuros XB algorithm. Conclusions In general, results suggest that the Acuros XB algo- The new Acuros XB photon dose calculation engine is rithm is mature for clinical implementation and can tested for accuracy against Monte Carlo simulations in provide a valid and accurate alternative to Monte Carlo phantoms with simple geometrical heterogeneities in calculations. GAI [%] GAI [%] GAI [%] GAI [%] Pre Pre Pre Pre In In In In Post Post Post Post Edge-out Edge-out Edge-out Edge-out Edge-in Edge-in Edge-in Edge-in Axis-in Axis-in Axis-in Axis-in Axis-out GAI [%] Axis-out GAI [%] Axis-out GAI [%] Axis-out GAI [%] Pre Pre Pre Pre In In In In Post Post Post Post Edge-out Edge-out Edge-out Edge-out Edge-in Edge-in Edge-in Edge-in Axis-in Axis-in Axis-in Axis-in Axis-out GAI [%] Axis-out GAI [%] Axis-out GAI [%] Axis-out GAI [%] Pre Pre Pre Pre In In In In Post Post Post Post Edge- Edge- Edge-out Edge-out Edge-in Edge-in Edge-in Edge-in Axis-in Axis-in Axis-in Axis-in Axis-out Axis-out GAI [%] Axis-out GAI [%] Axis-out GAI [%] GAI [%] Pre Pre Pre Pre Bone Bone Bone Bone Air Air Air Air Adipose Adipose Adipose Adipose Cartilage Cartilage Cartilage Cartilage Post Post Post Post Fogliata et al. Radiation Oncology 2011, 6:82 Page 14 of 15 http://www.ro-journal.com/content/6/1/82 Table 2 Gamma Agreement Index GAI 6X 15X LF SF LF SF Dose to medium NormalLung Acuros XB 99.9 99.6 100.0 100.0 AAA 91.2 91.6 98.9 66.6 LightLung Acuros XB 85.6 90.1 81.5 85.5 AAA 47.9 6.3 18.4 1.6 Bone Acuros XB 99.9 99.8 100.0 100.0 AAA 62.2 41.8 100.0 100.0 Dose to water NormalLung Acuros XB 99.8 99.2 100.0 99.3 AAA 97.1 95.8 98.0 48.2 LightLung Acuros XB 83.5 90.5 83.8 90.0 AAA 34.8 6.1 12.2 1.6 Bone Acuros XB 89.0 75.8 90.5 72.7 AAA 79.8 97.4 18.3 12.6 Dose rescaled to water NormalLung Acuros XB 99.9 98.7 99.9 99.3 AAA 99.5 93.3 97.6 51.8 LightLung Acuros XB 81.0 88.6 78.6 85.6 AAA 29.9 6.0 7.8 1.6 Bone Acuros XB 99.9 99.8 99.9 99.9 AAA 100.0 100.0 100.0 100.0 3%, 3 mm thresholds in the area of the insert included in the beam in the plane parallel to the beam central axis passing through the isocentre. Benchmark dose: VMC++, test dose: AAA or Acuros XB version 10. LF = large field, SF = small field Additional material results of 2%, 2 mm (thin cross-hatching) and 3%, 3 mm (thick cross- hatching). In columns: Normal Lung, Light Lung, Bone, phantom B; in rows: SF and LF for 6X, SF and LF for 15X. Additional file 1: DD in water for phantom A. Depth dose curves (DD) at -4 cm off-axis. Dose to water calculations for VMC++, Acuros XB Additional file 8: GAI for dose rescaled to water. Histograms of the version 10, and AAA in phantom A. In columns: Normal Lung, Light GAI. Global gamma calculation for each sector of phantoms A and B, for Lung, Bone; in rows: SF and LF for 6X, SF and LF for 15X. dose rescaled to water calculations for Acuros XB version 10 (red horizontal hatching) and AAA (green diagonal hatching). Each bin Additional file 2: DD rescaled to water for phantom A. Depth dose represents the two threshold results of 2%, 2 mm (thin cross-hatching) curves (DD) at -4 cm off-axis. Dose rescaled to water calculations for VMC and 3%, 3 mm (thick cross-hatching). In columns: Normal Lung, Light ++, Acuros XB version 10, and AAA in phantom A. In columns: Normal Lung, Bone, phantom B; in rows: SF and LF for 6X, SF and LF for 15X. Lung, Light Lung, Bone; in rows: SF and LF for 6X, SF and LF for 15X. Additional file 3: Dose profiles to water for phantom A. Profiles at mid-depth of the heterogeneity insert. Dose to water calculations for VMC ++, Acuros XB version 10, and AAA in phantom A. In columns: Normal Acknowledgements Lung, Light Lung, Bone; in rows: SF and LF for 6X, SF and LF for 15X. The authors thank the whole Varian Medical System group in Helsinki, Additional file 4: Dose profiles rescaled to water for phantom A. Finland, especially Pekka Uusitalo, Tuomas Torsti, Laura Korhonen, Viljo Petaja Profiles at mid-depth of the heterogeneity insert. Dose rescaled to water and Stephen Thompson for the fruitful discussions arising during the calculations for VMC++, Acuros XB version 10, and AAA in phantom A. In evaluation and testing phase of the Acuros XB through its various columns: Normal Lung, Light Lung, Bone; in rows: SF and LF for 6X, SF developing versions. and LF for 15X. Additional file 5: DD to water for phantom B. Depth dose curves Authors’ contributions (DD) at beam central axis. Dose to water calculations for VMC++, Acuros AF and LC coordinated the entire study. Data acquisition and 1-D analysis XB version 10, and AAA in phantom B. In columns: SF, LF; in rows: 6X, were conducted by AF, GN and LC. 2-D analysis was done by AC and EV. 15X. The manuscript was prepared by AF. All authors read and approved the final manuscript. Additional file 6: DD rescaled to water for phantom B. Depth dose curves (DD) at beam central axis. Dose rescaled to water calculations for Competing interests VMC++, Acuros XB version 10, and AAA in phantom B. In columns: SF, The present work was partially supported by a Grant from Varian Medical LF; in rows: 6X, 15X. Systems, Palo Alto, CA, USA. Additional file 7: GAI for dose to water. Histograms of the GAI. Global Dr. L. Cozzi acts as Scientific Advisor to Varian Medical Systems and is Head gamma calculation for each sector of phantoms A and B, for dose to of Research and Technological Development to Oncology Institute of water calculations for Acuros XB version 10 (red horizontal hatching) and Southern Switzerland, IOSI, Bellinzona. AAA (green diagonal hatching). Each bin represents the two threshold Fogliata et al. Radiation Oncology 2011, 6:82 Page 15 of 15 http://www.ro-journal.com/content/6/1/82 Received: 23 May 2011 Accepted: 19 July 2011 Published: 19 July 2011 22. 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Radiation Oncology – Springer Journals
Published: Jul 19, 2011
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