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Motion compensation with a scanned ion beam: a technical feasibility study

Motion compensation with a scanned ion beam: a technical feasibility study Background: Intrafractional motion results in local over- and under-dosage in particle therapy with a scanned beam. Scanned beam delivery offers the possibility to compensate target motion by tracking with the treatment beam. Methods: Lateral motion components were compensated directly with the beam scanning system by adapting nominal beam positions according to the target motion. Longitudinal motion compensation to mitigate motion induced range changes was performed with a dedicated wedge system that adjusts effective particle energies at isocenter. Results: Lateral compensation performance was better than 1% for a homogeneous dose distribution when comparing irradiations of a stationary radiographic film and a moving film using motion compensation. The accuracy of longitudinal range compensation was well below 1 mm. Conclusion: Motion compensation with scanned particle beams is technically feasible with high precision. Background In a pilot project at Gesellschaft für Schwerionenforsc- In conformal radiotherapy, geometric margins are com- hung (GSI) [6-9], approximately 400 patients have been monly used to account for intra-fractional target motion treated with scanned carbon ion beams with the raster- [1,2]. These margins inevitably lead to inclusion of scan system [10]. For raster scanning, the target volume is healthy tissue in the treated volume. In intensity modu- divided in slices corresponding to equal ion energies. Irra- lated radiotherapy, additional motion effects arise due to diations are performed slice-by-slice. The required particle so called interplay effects [3-5]. Treatments are delivered energy is requested from the synchrotron for each slice. in small partial doses that only result in adequate total Within each slice, a narrow pencil beam is scanned on a dosage if they match as intended. In anatomy's eye view, virtual raster grid. To achieve the desired dose distribu- target motion leads to relative displacement of partial tion, the number of particles is optimized for each raster dose depositions and therefore results in local over- and position during treatment planning including biological under-dosage. effects [11-16]. The scanning progress is intensity control- Page 1 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 led. The carbon ion pencil beam is directed to the next dle energy. In a second experiment, six different particle raster position by a magnetic deflection system as soon as energies were requested in mixed order to test the func- the planned number of particles has been deposited. After tionality of the system for variable and alternating energy all points within a slice have been irradiated, the beam is modulations. The maximum difference in energy corre- aborted and the next energy level is requested from the sponded to a water equivalent range difference of 27 mm. accelerator. To date, only patients with tumors that are not Again, the energy modulation system was used to adapt subject to intra-fractional motion have been treated [7,17- the effective particle energy to a single range. 19]. For treatments with scanned particle beams, target motion would inevitably lead to local over- and under- 3D online motion compensation dosage due to the relative lateral motion between pencil Lateral motion compensation beam positions as well as possible motion induced The raster scanning process is controlled by the TCS. Beam changes in radiological depths. position as well as delivered number of particles are mon- itored in intervals of ~150 μs and ~10 μs respectively. The To treat moving targets, while maintaining the conformity standard TCS can adjust small deviations of the actual between target and treated volume as well as avoiding beam position via a fast feedback loop. Whenever the local over- and under-dosage, we are investigating and beam position has been measured, possible deviations are developing a system to adapt 3D pencil beam positions to fed back to the control of the scanning magnets to correct actual target positions in real time. Initially, simulation the beam position to the nominal position. Typically, studies were performed to investigate the potential of tar- deviations are within ± 0.5 mm and corrected after each get tracking with a scanned ion beam [4,20]. In beam's eye measurement cycle. The irradiation time for an individual view, lateral motion adaptation of pencil beam positions raster point is typically in the order of 5–10 ms. is feasible by applying offsets to the raster scanner settings. Real time energy adaptation to compensate changes in Several processes are running simultaneously in the TCS radiological depth with the synchrotron directly is not including monitoring of the beam intensity, the beam (yet) possible. Therefore online adaptation of particle position, and the raster scanner magnet settings. The indi- ranges has to be performed with an additional, dedicated vidual processes communicate via a control loop as well energy modulation system. One of the possibilities is to as shared memory. For motion compensation, adaptation use a dedicated absorber wedge system [21]. of lateral pencil beam positions was implemented by dynamically changing the nominal values of the beam Prototype systems for lateral as well as longitudinal target positions in shared memory. As soon as the nominal val- tracking with a scanned ion beam have been developed. ues have been changed, the feedback loop adjusts the Experimental results are presented to demonstrate the fea- beam position accordingly. A dedicated, additional proc- sibility of target tracking with a scanned ion beam and to ess running on the TCS receives displacement vectors and show the performance of the individual prototype track- then changes the nominal beam positions in shared mem- ing sub-systems. ory accordingly. In order to avoid hardware changes within the TCS for the prototype setup, a standard net- work connection (100 Hz) was used to transmit displace- Methods Simulation of target motion ment vectors to the TCS. The actual displacement vector is Lateral target motion orthogonal to the beam direction added to the stationary nominal raster point position to was achieved with a three-axes positioning table. A radio- compute the new, dynamic nominal position. graphic film was mounted on the table as detector. The motion was sinusoidal with a period of ~10 s and ampli- Longitudinal motion compensation tudes of ± 15 mm in horizontal as well as vertical direc- To perform motion compensation in longitudinal direc- tion. No external motion monitoring device was used, tion, the energy of individual pencil beams has to be instead table motion was continuously measured with adjusted in quasi real time. Because fast active energy var- encoders. Target displacements were evaluated from iation with the accelerator is not possible, a passive energy encoder data and sent directly to the therapy control sys- modulation system was developed and installed between tem (TCS) for beam adaptation during irradiations. beam exit window and isocenter [21]. The system consists of two opposing lucite wedge absorbers that are mounted To simulate motion induced variations in particle range, on linear motor drives orthogonal to the beam direction different particle energies were requested from the syn- (figure 1). By moving the wedges apart (together) with the chrotron. In a first experiment, three different particle linear motors, the thickness of absorber material in the energies were requested from the accelerator repeatedly in beam path can be decreased (increased) to adapt the effec- fixed order. The energy modulation system was used to tive beam range at isocenter fast and continuously. The adapt the effective particle energy at isocenter to the mid- system has an active compensation area of 120 × 150 Page 2 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 En Figure 1 ergy modulation system Energy modulation system. Two opposing wedge shaped absorbers are mounted on linear motors between beam exit window and isocenter to continuously adjust the effective energy; left: schematic drawing, right: photographs in oblique side view (upper) and top view (lower). mm . The absorber wedges were designed to provide FIPS Plus software for film dosimetry (PTW Freiburg) with homogeneous range adaptation within the active area by a spatial resolution of 1 mm. Based on the film responses, adequate overlap. If the treatment field exceeds the absorbed doses were calculated according to Bathelt et al dimension of the active area in the horizontal direction of [23] and Spielberger at al [24]. Simple treatment plans wedge motion both wedges can be moved synchronously were optimized to deliver homogeneous, quadratic dose to provide adequate range adaptation. The total wedge distributions as well as line patterns. Geometric properties thickness of the prototype system corresponds to a maxi- of motion compensation were assessed from the line pat- mum water equivalent range variation of ± 49.4 mm terns. For quadratic fields, the homogeneity index H was which should exceed the maximum clinically required computed to compare dose distributions quantitatively: range adaptation. ∑−() D D Measurement and analysis of dose distributions i (1) Different detectors were used to measure dose distribu- H=− 1 D N−1 tions: planar radiographic films for lateral 2D dose distri- butions and a range telescope for longitudinal 1D depth with D dose to each individual pixel, N number of pixels dose distributions [22]. within the target area, and mean dose within the target area. Radiographic films (Kodak X-Omat V) were developed with a Kodak M35 processing machine. The films were digitalized with a Kodak LS75 laser densitometer and the Page 3 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 The range telescope was used to measure depth dose dis- motion artifact (figure 3b; S5). Differences in relative dose tributions, so called Bragg peaks. The telescope consists of between the two experiments are within the precision of two parallel plate ionization chambers in front of and film measurements. behind a water tank of variable thickness [22,25,26]. Dur- ing the measurements, the thickness of the water tank was Longitudinal motion compensation increased in steps of 50 μm. The precision of longitudinal motion compensation is presented in figure 4. During irradiation, three different energy levels were adapted to the middle energy using the Results Lateral motion compensation energy modulation system. The inlay shows that the dif- Figure 2 shows film responses for a quadratic, homogene- ference to an individually measured depth dose distribu- ous field. Under motion, marked local over- as well as tion at the mean energy is ~0.1 mm. under-dosage are apparent and relevant dose is deposited outside of the target area. Lateral motion compensation The performance of energy adaptation for 6 different restored the dose distribution on the moving film. In energy levels requested in random order from the acceler- comparison to the reference dose distribution, only small ator is shown in figure 5. The energy modulation system differences within the irradiated area are visible. Homoge- successfully restored a single, effective particle energy at neity indices were 0.969, 0.655, and 0.963 for the dose isocenter. Fluctuations around the reference depth dose distributions measured under stationary, moving, and distribution of ~2.5% on average (normalized to the motion compensated conditions respectively. Bragg peak) are mainly attributed to residual calibration uncertainties of the energy modulation system. Film responses for line patterns are shown in figure 3a. In contrast to the regular, parallel lines on the stationary Discussion film, heavily distorted patterns were measured with the The results of our feasibility study demonstrate that moving film. Motion compensation successfully restored motion compensation with scanned particle beams is fea- the line patterns. A small residual motion artifact is sible with high precision. Lateral as well as longitudinal present in the third line from top which was attributed to compensation were successfully performed during irradi- a sporadic communication delay between motion moni- ations. In a next step, both motion compensation sub-sys- toring and compensation due to communication via a tems have to be integrated in the therapy control system. standard network connection. Figure 3b presents line pro- Especially replacing standard network connections to files of the film responses for stationary and motion com- transmit compensation parameters should improve the pensated measurements. Positional differences of the reliability of the system. Furthermore, hardware improve- lines were on average 0.2 ± 0.2 mm. A maximum devia- ments of the energy modulation system for longitudinal tion of 1.6 mm was observed in the region of the residual Lateral motion compensation Figure 2 Lateral motion compensation. Dose distributions measured with radiographic films: stationary, moving, and moving using lateral motion compensation. Homogeneity indices were 0.969, 0.655, and 0.963 respectively. Page 4 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 Geometri Figure 3 cal performance of lateral motion compensation Geometrical performance of lateral motion compensation. a) Line patterns irradiated on radiographic films: station- ary, moving, and moving using lateral motion compensation. b) Line profiles of the particle fluences in vertical direction at the positions indicated on the film measurements. range compensation should be investigated, and imple- least in vertical direction. In contrast, the horizontal mentation of motion monitoring has to be developed. dimension of the active area does not necessarily have to match the scan area. If the center of mass of the wedges Re-design of the wedge system for fast longitudinal follows the left-right motion of the ion beam during raster motion compensation is advisable since the thickness of scanning, an active area that is smaller than the maximum the wedges can most likely be reduced to the compensa- treatment area is sufficient. However, less wedge motion tion range required for patient treatments in order to and therefore reduced system performance is required if reduce lateral scattering as well as fragmentation of the the active area is sufficiently large to cover the complete primary particle beam [27-29]. Furthermore, the active scanning area. Detailed requirements on the compensa- area of the wedge system (120 × 150 mm ) does currently tion speed have to be derived from simulation studies, for not match the treatment area of the scanning system (200 example based on 4D computed tomography data [30- × 200 mm ). The wedge size thus has to be increased at 32]. Page 5 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 Since the particle range and thus the Bragg peak position are influenced by target motion and currently no motion monitoring system exists to determine changes in water- equivalent range a link to 4D treatment planning is required [47,48]. Motion states from 4DCT which are used to determine range changes could be detected by motion monitoring. Compensation vectors are then cal- culated during treatment planning and applied according to detected motion states. In case of motion irregularities or unknown motion states the treatment can be paused until the patient is back to normal breathing. Conclusion The results of our study demonstrate the high precision that is technically feasible for motion tracking with scanned particle beams. Lateral motion compensation Lon Figure 4 gitudinal motion compensation restored homogeneous dose distributions delivered to Longitudinal motion compensation. Bragg peaks meas- moving targets. Differences in dose uniformity between ured for three different energies and obtained by longitudinal motion compensation to the central energy. The inlay shows irradiation of a stationary radiographic film and a moving the sub-millimeter accuracy of longitudinal motion compen- film using motion compensation were below 1%. Longi- sation. tudinal compensation precision was well below 1 mm. Competing interests Another problem of motion tracking that has not yet been SOG and ER are now employed by Siemens Healthcare. solved adequately is precise monitoring of target motion. Research was performed while both were employed by To date, several different methods have been reported in GSI. the literature. Currently, the most promising technique seems to be fluoroscopic motion detection because target Authors' contributions motion is imaged directly [33-38]. Other techniques that All authors contributed to the design of the prototype sys- monitor external surface motion have to be evaluated tem and the conceptual design of the study. Furthermore, regarding the accuracy to derive target positions [39-46]. SOG performed measurements, analyzed data, and drafted the manuscript. CB and ER supported measure- ments, analyzed data, and revised the manuscript. TH and GK improved the conceptual design and revised the man- uscript. All authors read and approved the final manu- script. References 1. ICRU: Report 50. Bethesda, Md, USA, International Commission on Radiation Units and Measurements; 1993. 2. ICRU: Report 62. Bethesda, Md, USA, International Commission on Radiation Units and Measurements; 1999. 3. Phillips MH, Pedroni E, Blattmann H, Boehringer T, Coray A, Scheib S: Effects of respiratory motion on dose uniformity with a charged particle scanning method. Phys Med Biol 1992, 37:223-233. 4. Grozinger SO, Rietzel E, Li Q, Bert C, Haberer T, Kraft G: Simula- tions to design an online motion compensation system for scanned particle beams. Phys Med Biol 2006, 51:3517-3531. 5. Bert C, Grözinger SO, Rietzel E: Quantification of interplay effects of scanned particle beams and moving targets. Phys Med Biol 2008, 53:2253-2265. 6. Kraft G: Tumor Therapy with Heavy Charged Particles. Prog Perfor energi Figure 5 es mance of longitudinal motion compensation for mixed Part Nucl Phys 2000, 45:473. Performance of longitudinal motion compensation 7. Debus J, Haberer T, Schulz-Ertner D, Jakel O, Wenz F, Enghardt W, for mixed energies. Longitudinal motion compensation for Schlegel W, Kraft G, Wannenmacher M: Carbon ion irradiation of skull base tumors at GSI. 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Phys Med Biol 2001, 46:1101-1116. kamoto T, Koyanagi T, Miyasaka K: Use of an implanted marker 17. Schulz-Ertner D, Nikoghosyan A, Thilmann C, Haberer T, Jäkel O, and real-time tracking of the marker for the positioning of Karger C, Kraft G, Wannenmacher M, Debus J: Results of carbon prostate and bladder cancers. Int J Radiat Oncol 2000, ion radiotherapy in 152 patients. Int J Radiat Oncol 2004, 48:1591-1597. 58:631-640. 37. Schweikard A, Shiomi H, Adler J: Respiration tracking in radio- 18. Schulz-Ertner D, Nikoghosyan A, Hof H, Didinger B, Combs SE, Jakel surgery. Med Phys 2004, 31:2738-2741. O, Karger CP, Edler L, Debus J: Carbon ion radiotherapy of skull 38. Rietzel E, Rosenthal SJ, Gierga DP, Willet CG, Chen GT: Moving base chondrosarcomas. Int J Radiat Oncol Biol Phys 2007, targets: detection and tracking of internal organ motion for 67:171-177. treatment planning and patient set-up. Radiother Oncol 2004, 19. 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Int J Radiat Oncol 2004, 60:579. tigations of the response of films to heavy-ion irradiation. 44. Bert C, Metheany KG, Doppke K, Chen GT: A phantom evalua- Phys Med Biol 2001, 46:2889-2897. tion of a stereo-vision surface imaging system for radiother- 25. Schardt D, Stelzer H, Junk H, Arndt U: Bragg curve measure- apy patient setup. Med Phys 2005, 32:2753-2762. ments with ionisation chambers. Grundinger, U. 336. Darm- 45. Li XA, Stepaniak C, Gore E: Technical and dosimetric aspects of stadt, Gesellschaft für Schwerionenforschung mbH. GSI Scientific Report respiratory gating using a pressure-sensor motion monitor- 1992 1993. ing system. Med Phys 2006, 33:145-154. 26. Rietzel E, Schardt D, Haberer T: Range accuracy in carbon ion 46. Willoughby TR, Forbes AR, Buchholz D, Langen KM, Wagner TH, treatment planning based on CT-calibration with real tissue Zeidan OA, Kupelian PA, Meeks SL: Evaluation of an infrared samples. Radiat Oncol 2007, 2:14. camera and X-ray system using implanted fiducials in 27. Schardt D, Schall I, Geissel H, Irnich H, Kraft G, Magel A, Mohar MF, patients with lung tumors for gated radiation therapy. Int J Munzenberg G, Nickel F, Scheidenberger C, Schwab W, Sihver L: Radiat Oncol Biol Phys 2006, 66:568-575. Nuclear fragmentation of high-energy heavy-ion beams in 47. Rietzel E, Chen GTY, Choi NC, Willet CG: Four-dimensional water. Adv Space Res 1996, 17:87-94. image-based treatment planning: Target volume segmenta- 28. Schall I, Schardt D, Geissel H, Irnich H, Kankeleit E, Kraft G, Magel A, tion and dose calculation in the presence of respiratory Mohar MF, Mnnzenberg G, Nickel F, Scheidenberger C, Schwab W: motion. Int J Radiat Oncol 2005, 61:1535-1550. Charge-changing nuclear reactions of relativistic light-ion 48. Bert C, Rietzel E: 4D treatment planning for scanned ion beams (5 <= Z <= 10) passing through thick absorbers. Nucl beams. Radiat Oncol 2007, 2:. Instrum Meth B 1996, 117:221-234. 29. Gunzert-Marx K, Iwase H, Schardt D, Simon RS: Secondary beam fragments produced by 200 MeVu(-1) C-12 ions in water and their dose contributions in carbon ion radiotherapy. New Jour- nal of Physics 2008, 10:. 30. Ford EC, Mageras GS, Yorke E, Ling CC: Respiration-correlated spiral CT: A method of measuring respiratory-induced ana- tomic motion for radiation treatment planning. Med Phys 2003, 30:88-97. 31. Vedam SS, Keall PJ, Kini VR, Mostafavi H, Shukla HP, Mohan R: Acquiring a four-dimensional computed tomography data- set using an external respiratory signal. Phys Med Biol 2003, 48:45-62. Page 7 of 7 (page number not for citation purposes) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Oncology Springer Journals

Motion compensation with a scanned ion beam: a technical feasibility study

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Springer Journals
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Copyright © 2008 by Grözinger et al; licensee BioMed Central Ltd.
Subject
Medicine & Public Health; Oncology; Radiotherapy
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1748-717X
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10.1186/1748-717X-3-34
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18854012
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Abstract

Background: Intrafractional motion results in local over- and under-dosage in particle therapy with a scanned beam. Scanned beam delivery offers the possibility to compensate target motion by tracking with the treatment beam. Methods: Lateral motion components were compensated directly with the beam scanning system by adapting nominal beam positions according to the target motion. Longitudinal motion compensation to mitigate motion induced range changes was performed with a dedicated wedge system that adjusts effective particle energies at isocenter. Results: Lateral compensation performance was better than 1% for a homogeneous dose distribution when comparing irradiations of a stationary radiographic film and a moving film using motion compensation. The accuracy of longitudinal range compensation was well below 1 mm. Conclusion: Motion compensation with scanned particle beams is technically feasible with high precision. Background In a pilot project at Gesellschaft für Schwerionenforsc- In conformal radiotherapy, geometric margins are com- hung (GSI) [6-9], approximately 400 patients have been monly used to account for intra-fractional target motion treated with scanned carbon ion beams with the raster- [1,2]. These margins inevitably lead to inclusion of scan system [10]. For raster scanning, the target volume is healthy tissue in the treated volume. In intensity modu- divided in slices corresponding to equal ion energies. Irra- lated radiotherapy, additional motion effects arise due to diations are performed slice-by-slice. The required particle so called interplay effects [3-5]. Treatments are delivered energy is requested from the synchrotron for each slice. in small partial doses that only result in adequate total Within each slice, a narrow pencil beam is scanned on a dosage if they match as intended. In anatomy's eye view, virtual raster grid. To achieve the desired dose distribu- target motion leads to relative displacement of partial tion, the number of particles is optimized for each raster dose depositions and therefore results in local over- and position during treatment planning including biological under-dosage. effects [11-16]. The scanning progress is intensity control- Page 1 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 led. The carbon ion pencil beam is directed to the next dle energy. In a second experiment, six different particle raster position by a magnetic deflection system as soon as energies were requested in mixed order to test the func- the planned number of particles has been deposited. After tionality of the system for variable and alternating energy all points within a slice have been irradiated, the beam is modulations. The maximum difference in energy corre- aborted and the next energy level is requested from the sponded to a water equivalent range difference of 27 mm. accelerator. To date, only patients with tumors that are not Again, the energy modulation system was used to adapt subject to intra-fractional motion have been treated [7,17- the effective particle energy to a single range. 19]. For treatments with scanned particle beams, target motion would inevitably lead to local over- and under- 3D online motion compensation dosage due to the relative lateral motion between pencil Lateral motion compensation beam positions as well as possible motion induced The raster scanning process is controlled by the TCS. Beam changes in radiological depths. position as well as delivered number of particles are mon- itored in intervals of ~150 μs and ~10 μs respectively. The To treat moving targets, while maintaining the conformity standard TCS can adjust small deviations of the actual between target and treated volume as well as avoiding beam position via a fast feedback loop. Whenever the local over- and under-dosage, we are investigating and beam position has been measured, possible deviations are developing a system to adapt 3D pencil beam positions to fed back to the control of the scanning magnets to correct actual target positions in real time. Initially, simulation the beam position to the nominal position. Typically, studies were performed to investigate the potential of tar- deviations are within ± 0.5 mm and corrected after each get tracking with a scanned ion beam [4,20]. In beam's eye measurement cycle. The irradiation time for an individual view, lateral motion adaptation of pencil beam positions raster point is typically in the order of 5–10 ms. is feasible by applying offsets to the raster scanner settings. Real time energy adaptation to compensate changes in Several processes are running simultaneously in the TCS radiological depth with the synchrotron directly is not including monitoring of the beam intensity, the beam (yet) possible. Therefore online adaptation of particle position, and the raster scanner magnet settings. The indi- ranges has to be performed with an additional, dedicated vidual processes communicate via a control loop as well energy modulation system. One of the possibilities is to as shared memory. For motion compensation, adaptation use a dedicated absorber wedge system [21]. of lateral pencil beam positions was implemented by dynamically changing the nominal values of the beam Prototype systems for lateral as well as longitudinal target positions in shared memory. As soon as the nominal val- tracking with a scanned ion beam have been developed. ues have been changed, the feedback loop adjusts the Experimental results are presented to demonstrate the fea- beam position accordingly. A dedicated, additional proc- sibility of target tracking with a scanned ion beam and to ess running on the TCS receives displacement vectors and show the performance of the individual prototype track- then changes the nominal beam positions in shared mem- ing sub-systems. ory accordingly. In order to avoid hardware changes within the TCS for the prototype setup, a standard net- work connection (100 Hz) was used to transmit displace- Methods Simulation of target motion ment vectors to the TCS. The actual displacement vector is Lateral target motion orthogonal to the beam direction added to the stationary nominal raster point position to was achieved with a three-axes positioning table. A radio- compute the new, dynamic nominal position. graphic film was mounted on the table as detector. The motion was sinusoidal with a period of ~10 s and ampli- Longitudinal motion compensation tudes of ± 15 mm in horizontal as well as vertical direc- To perform motion compensation in longitudinal direc- tion. No external motion monitoring device was used, tion, the energy of individual pencil beams has to be instead table motion was continuously measured with adjusted in quasi real time. Because fast active energy var- encoders. Target displacements were evaluated from iation with the accelerator is not possible, a passive energy encoder data and sent directly to the therapy control sys- modulation system was developed and installed between tem (TCS) for beam adaptation during irradiations. beam exit window and isocenter [21]. The system consists of two opposing lucite wedge absorbers that are mounted To simulate motion induced variations in particle range, on linear motor drives orthogonal to the beam direction different particle energies were requested from the syn- (figure 1). By moving the wedges apart (together) with the chrotron. In a first experiment, three different particle linear motors, the thickness of absorber material in the energies were requested from the accelerator repeatedly in beam path can be decreased (increased) to adapt the effec- fixed order. The energy modulation system was used to tive beam range at isocenter fast and continuously. The adapt the effective particle energy at isocenter to the mid- system has an active compensation area of 120 × 150 Page 2 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 En Figure 1 ergy modulation system Energy modulation system. Two opposing wedge shaped absorbers are mounted on linear motors between beam exit window and isocenter to continuously adjust the effective energy; left: schematic drawing, right: photographs in oblique side view (upper) and top view (lower). mm . The absorber wedges were designed to provide FIPS Plus software for film dosimetry (PTW Freiburg) with homogeneous range adaptation within the active area by a spatial resolution of 1 mm. Based on the film responses, adequate overlap. If the treatment field exceeds the absorbed doses were calculated according to Bathelt et al dimension of the active area in the horizontal direction of [23] and Spielberger at al [24]. Simple treatment plans wedge motion both wedges can be moved synchronously were optimized to deliver homogeneous, quadratic dose to provide adequate range adaptation. The total wedge distributions as well as line patterns. Geometric properties thickness of the prototype system corresponds to a maxi- of motion compensation were assessed from the line pat- mum water equivalent range variation of ± 49.4 mm terns. For quadratic fields, the homogeneity index H was which should exceed the maximum clinically required computed to compare dose distributions quantitatively: range adaptation. ∑−() D D Measurement and analysis of dose distributions i (1) Different detectors were used to measure dose distribu- H=− 1 D N−1 tions: planar radiographic films for lateral 2D dose distri- butions and a range telescope for longitudinal 1D depth with D dose to each individual pixel, N number of pixels dose distributions [22]. within the target area, and mean dose within the target area. Radiographic films (Kodak X-Omat V) were developed with a Kodak M35 processing machine. The films were digitalized with a Kodak LS75 laser densitometer and the Page 3 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 The range telescope was used to measure depth dose dis- motion artifact (figure 3b; S5). Differences in relative dose tributions, so called Bragg peaks. The telescope consists of between the two experiments are within the precision of two parallel plate ionization chambers in front of and film measurements. behind a water tank of variable thickness [22,25,26]. Dur- ing the measurements, the thickness of the water tank was Longitudinal motion compensation increased in steps of 50 μm. The precision of longitudinal motion compensation is presented in figure 4. During irradiation, three different energy levels were adapted to the middle energy using the Results Lateral motion compensation energy modulation system. The inlay shows that the dif- Figure 2 shows film responses for a quadratic, homogene- ference to an individually measured depth dose distribu- ous field. Under motion, marked local over- as well as tion at the mean energy is ~0.1 mm. under-dosage are apparent and relevant dose is deposited outside of the target area. Lateral motion compensation The performance of energy adaptation for 6 different restored the dose distribution on the moving film. In energy levels requested in random order from the acceler- comparison to the reference dose distribution, only small ator is shown in figure 5. The energy modulation system differences within the irradiated area are visible. Homoge- successfully restored a single, effective particle energy at neity indices were 0.969, 0.655, and 0.963 for the dose isocenter. Fluctuations around the reference depth dose distributions measured under stationary, moving, and distribution of ~2.5% on average (normalized to the motion compensated conditions respectively. Bragg peak) are mainly attributed to residual calibration uncertainties of the energy modulation system. Film responses for line patterns are shown in figure 3a. In contrast to the regular, parallel lines on the stationary Discussion film, heavily distorted patterns were measured with the The results of our feasibility study demonstrate that moving film. Motion compensation successfully restored motion compensation with scanned particle beams is fea- the line patterns. A small residual motion artifact is sible with high precision. Lateral as well as longitudinal present in the third line from top which was attributed to compensation were successfully performed during irradi- a sporadic communication delay between motion moni- ations. In a next step, both motion compensation sub-sys- toring and compensation due to communication via a tems have to be integrated in the therapy control system. standard network connection. Figure 3b presents line pro- Especially replacing standard network connections to files of the film responses for stationary and motion com- transmit compensation parameters should improve the pensated measurements. Positional differences of the reliability of the system. Furthermore, hardware improve- lines were on average 0.2 ± 0.2 mm. A maximum devia- ments of the energy modulation system for longitudinal tion of 1.6 mm was observed in the region of the residual Lateral motion compensation Figure 2 Lateral motion compensation. Dose distributions measured with radiographic films: stationary, moving, and moving using lateral motion compensation. Homogeneity indices were 0.969, 0.655, and 0.963 respectively. Page 4 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 Geometri Figure 3 cal performance of lateral motion compensation Geometrical performance of lateral motion compensation. a) Line patterns irradiated on radiographic films: station- ary, moving, and moving using lateral motion compensation. b) Line profiles of the particle fluences in vertical direction at the positions indicated on the film measurements. range compensation should be investigated, and imple- least in vertical direction. In contrast, the horizontal mentation of motion monitoring has to be developed. dimension of the active area does not necessarily have to match the scan area. If the center of mass of the wedges Re-design of the wedge system for fast longitudinal follows the left-right motion of the ion beam during raster motion compensation is advisable since the thickness of scanning, an active area that is smaller than the maximum the wedges can most likely be reduced to the compensa- treatment area is sufficient. However, less wedge motion tion range required for patient treatments in order to and therefore reduced system performance is required if reduce lateral scattering as well as fragmentation of the the active area is sufficiently large to cover the complete primary particle beam [27-29]. Furthermore, the active scanning area. Detailed requirements on the compensa- area of the wedge system (120 × 150 mm ) does currently tion speed have to be derived from simulation studies, for not match the treatment area of the scanning system (200 example based on 4D computed tomography data [30- × 200 mm ). The wedge size thus has to be increased at 32]. Page 5 of 7 (page number not for citation purposes) Radiation Oncology 2008, 3:34 http://www.ro-journal.com/content/3/1/34 Since the particle range and thus the Bragg peak position are influenced by target motion and currently no motion monitoring system exists to determine changes in water- equivalent range a link to 4D treatment planning is required [47,48]. Motion states from 4DCT which are used to determine range changes could be detected by motion monitoring. Compensation vectors are then cal- culated during treatment planning and applied according to detected motion states. In case of motion irregularities or unknown motion states the treatment can be paused until the patient is back to normal breathing. Conclusion The results of our study demonstrate the high precision that is technically feasible for motion tracking with scanned particle beams. Lateral motion compensation Lon Figure 4 gitudinal motion compensation restored homogeneous dose distributions delivered to Longitudinal motion compensation. Bragg peaks meas- moving targets. Differences in dose uniformity between ured for three different energies and obtained by longitudinal motion compensation to the central energy. The inlay shows irradiation of a stationary radiographic film and a moving the sub-millimeter accuracy of longitudinal motion compen- film using motion compensation were below 1%. Longi- sation. tudinal compensation precision was well below 1 mm. Competing interests Another problem of motion tracking that has not yet been SOG and ER are now employed by Siemens Healthcare. solved adequately is precise monitoring of target motion. Research was performed while both were employed by To date, several different methods have been reported in GSI. the literature. 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Radiation OncologySpringer Journals

Published: Oct 14, 2008

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