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172 EFFICIENT MONTE CARLO BASED ELECTRON BEAM MODEL FOR MERT USING A PHOTON MLC
S80 subtracted. The measurements showed that the SRAM detector may be a candidate for neutron dosimetry in heavy ion therapy. In July 2011 further measurements were conducted in a 200 MeV/u carbon ion beam at GSI with a water phantom. Detectors were placed at several angles and lateral distances from the central beam axis. The next generation of detectors will include an integrated charged particle veto on the detector in front and behind the SRAM chips. *Corresponding author: [email protected] 171 DO CHANGES IN NASAL CAVITY FILLING SIGNIFICANTLY AFFECT PROTON TREATMENTS? M. Verena, A. Bolsi, F. Albertini, A. Lomax Center for Proton Therapy, PSI The precision of proton radiotherapy strongly depends on the accuracy of the range calculation in the patient. Many factors can affect this calculation, including anatomical variation during treatment. In this work we have retrospectively estimated the influence of varying maxillary sinus filling on clinical proton dose distributions. We have considered three patients under treatment at PSI with tumors involving the sinuses and nasal cavities: an adenoidal carcinoma (patient A), a chondrosarcoma (patient B) and a giant cell tumour in the spheno-ethmoidal region (patient C). Dose prescriptions for target volumes and dose limits to the OAR (typically brainstem and optic structures) were indication specific and followed PSI standard treatment protocols. For all the patients, complete treatments were divided into two or more series, with IMPT (Intensity Modulated Proton Therapy) plans being delivered from the beginning or after the first series. First series plans were constructed of three fields, one lateral and superior-lateral obliques. Second series (IMPT) were planned using four quasi coplanar fields, consisting of two posterior-lateral and two anteriorlateral fields. For all cases, some significant changes in the filling of the nasal cavities were observed during the treatment period and new planning CTs acquired as necessary (one or more per patient). The nominal plans, optimized on the original planning CT, have then been recalculated on the new CT data sets in order to evaluate the dosimetric impact of these changes in the nasal cavities. In addition, we have investigated two worst cases scenarios, which define the maximum detrimental effect on the dose distributions in case of cavities being completely filled (fluid) or completely empty (air). For these, the nasal cavities have been delineated and the CT HU values within these structures manually set to 20 and -1000 respectively. The nominal plans have then also been recalculated two CT data sets. In all cases, the resultant dose distributions have been compared to the nominal ones through the use of Dose Volume Histograms (DVH) comparison and analysis of local dose differences. Although reduced filling leads to local dose overshoots, and increased filling to under-shoot, the changes in the DVHs for CTV and OAR’s were not found to be significant (<5%), particularly when assessing the “realistic” case of dose recalculation on the repeat CT’s. Indeed, although the effects were larger for the two worse cases scenario’s (fully filled or
ICTR-PHE 2012 completely empty), still the observed effects on DVH’s were minimal. On the other hand, cold and hot spots (up to 50% when the two extreme scenarios are compared) were observed, particularly behind the cavities, but these were generally located in noncritical normal tissues (see figure 1). In particular, in case of empty cavities, the portion of healthy tissue irradiated at high doses (100% and 110% of the prescription) was substantially higher (as reported in the table, for the example case). Dose to OARs (optical structures, brainstem, inner ears and parotid glands) show no variation between the different plans, as reported only for the brainstem in Table 1, for the example case. This study implies that even major changes in the nasal cavity filling may not affect proton plan quality as much as may be expected. However beam directions should be carefully selected to minimize the path through the cavities. Repeated CT's are very helpful to estimate the local dose distribution changes and to evaluate if re-planning is necessary. For this reason, for patients with target volumes involving cavities which may be subject to density changes, adequate and regular monitoring of the filling of these cavities should be performed during the treatment course.
172 EFFICIENT MONTE CARLO BASED ELECTRON BEAM MODEL FOR MERT USING A PHOTON MLC D. Henzen1, P. Manser1, D. Frei1, W. Volken1, H. Neuenschwander2, E. Born1, M. Fix1 1 Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital and University of Bern, Switzerland 2 Clinic for Radiation Oncology, Lindenhofspital Bern, Switzerland Purpose: In comparison with the standard treatment for superficial tumors, modulated electron radiotherapy (MERT) could lead to reduced doses in normal tissue and organs at risk. One approach for the MERT delivery is using a photon multi leaf collimator (MLC), which on the other hand requires a beam model for treatment planning purposes. In this work, an efficient beam model for Varian linear accelerators equipped with the Millennium 120-leaf MLC has been developed to accurately reproduce the beam characteristics of the linear accelerator head.
S81 Material and Methods: In order to obtain a flexible but still efficient beam model it has been divided into two parts: a patient independent part including the linear accelerator head above the MLC and a patient depending part covering the photon MLC. The patient independent part consists of a main diverging photon and electron source which represent the particles coming from the scattering foil and a photon and electron line source located at the secondary collimator jaws representing the head scatter. This part is used to reproduce the beam directly above the MLC for two settings of the secondary collimator jaws: a 35x35 cm2 allowing no MLC-leaf over-travel and a 15x35 cm2 allowing full MLC-leaf over-travel in order to provide a broad range of possible MLC shaped field sizes. The patient dependent part, i.e. the radiation transport through the MLC, is carried out using Monte Carlo (MC) methods. The MC transport code EGSnrc [1] has been used for benchmark purposes. However, in order to be computationally efficient an in-house developed MC transport for photons has been extended by means of implementing charged particle interactions and a simplified model for multiple scattering. Finally, the beam model has been connected with the macro MC algorithm [2] for dose calculation purposes. A commissioning procedure for the beam model has been developed using MC data and measurements as input. In order to verify and validate the beam model, calculated and measured dose distributions in units of cGy/MU and at a source to surface distance of 70 cm have been compared for the field sizes without MLC as well as for several MLC shaped fields and electron energies ranging from 4 to 22 MeV. Results: For fields shaped by the secondary collimator jaws (MLC retracted) calculated and measured absolute depth dose curves agree within 1% or 1 mm for all field sizes and energies. Calculated and measured absolute dose profiles at several depths in water generally agree to within 2% or 2 mm for all situations considered. The comparison between the calculated dose distributions for the MLC shaped fields and the corresponding measurements showed a general agreement within 1% or 1 mm for the depth dose curves and 2% or 2 mm for dose profiles at several depths in water. Conclusion: The results of the dose comparison using the newly developed beam model suggest that the dose distributions for electron beams shaped by a photon MLC can be calculated efficiently and accurately. The next step will be to investigate the feasibility and the benefits of MERT for clinical cases. This work was supported by Varian Medical Systems. References [1] I. Kawrakow, E. Mainegra-Hing, D.W.O. Rogers, F. Tessier and B.R.B. Walters, The EGSnrc Code System: Monte Carlo Simulation of Electron and Photon Transport, NRC Report PIRS-701, Ottawa, Canada 2009 [2] H. Neuenschwander and E.J. Born, A macro Monte Carlo method for electron beam dose calculations, Phys Med Biol 37, 107-25 (1992). 173 TARGET VOLUME OPTIMIZATION FOR PROSTATE CANCER TREATMENT IN CARBON ION RADIATION THERAPY IN THE PRESENCE OF INTERFRACTIONAL MOTION
ICTR-PHE 2012 A. Rucinski1, Ch. Bert3, S. Ecker2, M. Ellerbrock2, T. Haberer2, G. Habl1, K. Herfarth1, O. Jäkel1, F. Sterzing1 1 Department of Radiation Oncology, University of Heidelberg, Heidelberg, Germany 2 Heidelberg Ion Beam Therapy Center, Heidelberg, Germany 3 GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany Prostate cancer is the predominant cancer in men in developed countries. Carbon ion therapy could be an efficient method for treating this indication due to its high conformity and radiobiological effectiveness. However, the high conformity in particle therapy is challenged by organ motion because density variations in the beam path have a larger influence on the target dose as compared to photons. The tendency to apply a hypofractionated prostate treatment protocol emphasizes this problem. In photon therapy two approaches of pre-treatment registration are applied: bony anatomy or soft tissue registration, but the impact of the registration method on the quality of photon treatment is low (Kalz et al. 2008). In particle therapy the selection of bony anatomy pre-treatment registration seems to be more appropriate for treatment because density variations in the beam path appear not to have a significant impact on dose distribution. However, because of interfractional target motion additional extension of the clinical target volume is needed to ensure homogeneous target coverage (Rietzel, Bert, 2010). Although the application of soft tissue registration may mitigate impact of the target motion on the dose distribution, anatomy variations in the beam path may cause relevant dose shifts in beam direction. In this study we analyze treatment plan robustness concerning interfractional target motion, selection of registration method, motion induced density variations in the beam path and potential rectal toxicity. The aim of this treatment planning study is to develop a target volume definition method which provides the greatest treatment plan robustness. In the presented study Megavoltage Computed Tomography (MVCT) data acquired on a daily basis from a TomoTherapy unit from the Radiation Oncology Department at the Heidelberg University Clinic were used. For dose optimization and analysis of recalculated dose distributions all MVCT images were entirely contoured. The clinical target volume was defined according to world-wide commissioned segmentation protocol, as the volume of prostate extended by a six millimetres isotropic margin and including seminal vesicles. The treatment plans were optimized on selected MVCT images, separately for bony anatomy and soft tissue registration method. Independent treatment plans were optimized for the different target volume definition methods. The optimization and recalculation was performed with the TRiP Package (Treatment Planning for Particles, Krämer et al. 2000) successfully used in the Carbon Ion Therapy Pilot Project hosted at GSI, Darmstadt, Germany. Treatment plan recalculations performed on daily MVCT images contain information about the impact of target motion on dose distribution and indicate on anatomical variations. In order to perform assessment of recalculated therapy plans we analyzed:
ICTR-PHE 2012 completely empty), still the observed effects on DVH’s were minimal. On the other hand, cold and hot spots (up to 50% when the two extreme scenarios are compared) were observed, particularly behind the cavities, but these were generally located in noncritical normal tissues (see figure 1). In particular, in case of empty cavities, the portion of healthy tissue irradiated at high doses (100% and 110% of the prescription) was substantially higher (as reported in the table, for the example case). Dose to OARs (optical structures, brainstem, inner ears and parotid glands) show no variation between the different plans, as reported only for the brainstem in Table 1, for the example case. This study implies that even major changes in the nasal cavity filling may not affect proton plan quality as much as may be expected. However beam directions should be carefully selected to minimize the path through the cavities. Repeated CT's are very helpful to estimate the local dose distribution changes and to evaluate if re-planning is necessary. For this reason, for patients with target volumes involving cavities which may be subject to density changes, adequate and regular monitoring of the filling of these cavities should be performed during the treatment course.
172 EFFICIENT MONTE CARLO BASED ELECTRON BEAM MODEL FOR MERT USING A PHOTON MLC D. Henzen1, P. Manser1, D. Frei1, W. Volken1, H. Neuenschwander2, E. Born1, M. Fix1 1 Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital and University of Bern, Switzerland 2 Clinic for Radiation Oncology, Lindenhofspital Bern, Switzerland Purpose: In comparison with the standard treatment for superficial tumors, modulated electron radiotherapy (MERT) could lead to reduced doses in normal tissue and organs at risk. One approach for the MERT delivery is using a photon multi leaf collimator (MLC), which on the other hand requires a beam model for treatment planning purposes. In this work, an efficient beam model for Varian linear accelerators equipped with the Millennium 120-leaf MLC has been developed to accurately reproduce the beam characteristics of the linear accelerator head.
S81 Material and Methods: In order to obtain a flexible but still efficient beam model it has been divided into two parts: a patient independent part including the linear accelerator head above the MLC and a patient depending part covering the photon MLC. The patient independent part consists of a main diverging photon and electron source which represent the particles coming from the scattering foil and a photon and electron line source located at the secondary collimator jaws representing the head scatter. This part is used to reproduce the beam directly above the MLC for two settings of the secondary collimator jaws: a 35x35 cm2 allowing no MLC-leaf over-travel and a 15x35 cm2 allowing full MLC-leaf over-travel in order to provide a broad range of possible MLC shaped field sizes. The patient dependent part, i.e. the radiation transport through the MLC, is carried out using Monte Carlo (MC) methods. The MC transport code EGSnrc [1] has been used for benchmark purposes. However, in order to be computationally efficient an in-house developed MC transport for photons has been extended by means of implementing charged particle interactions and a simplified model for multiple scattering. Finally, the beam model has been connected with the macro MC algorithm [2] for dose calculation purposes. A commissioning procedure for the beam model has been developed using MC data and measurements as input. In order to verify and validate the beam model, calculated and measured dose distributions in units of cGy/MU and at a source to surface distance of 70 cm have been compared for the field sizes without MLC as well as for several MLC shaped fields and electron energies ranging from 4 to 22 MeV. Results: For fields shaped by the secondary collimator jaws (MLC retracted) calculated and measured absolute depth dose curves agree within 1% or 1 mm for all field sizes and energies. Calculated and measured absolute dose profiles at several depths in water generally agree to within 2% or 2 mm for all situations considered. The comparison between the calculated dose distributions for the MLC shaped fields and the corresponding measurements showed a general agreement within 1% or 1 mm for the depth dose curves and 2% or 2 mm for dose profiles at several depths in water. Conclusion: The results of the dose comparison using the newly developed beam model suggest that the dose distributions for electron beams shaped by a photon MLC can be calculated efficiently and accurately. The next step will be to investigate the feasibility and the benefits of MERT for clinical cases. This work was supported by Varian Medical Systems. References [1] I. Kawrakow, E. Mainegra-Hing, D.W.O. Rogers, F. Tessier and B.R.B. Walters, The EGSnrc Code System: Monte Carlo Simulation of Electron and Photon Transport, NRC Report PIRS-701, Ottawa, Canada 2009 [2] H. Neuenschwander and E.J. Born, A macro Monte Carlo method for electron beam dose calculations, Phys Med Biol 37, 107-25 (1992). 173 TARGET VOLUME OPTIMIZATION FOR PROSTATE CANCER TREATMENT IN CARBON ION RADIATION THERAPY IN THE PRESENCE OF INTERFRACTIONAL MOTION
ICTR-PHE 2012 A. Rucinski1, Ch. Bert3, S. Ecker2, M. Ellerbrock2, T. Haberer2, G. Habl1, K. Herfarth1, O. Jäkel1, F. Sterzing1 1 Department of Radiation Oncology, University of Heidelberg, Heidelberg, Germany 2 Heidelberg Ion Beam Therapy Center, Heidelberg, Germany 3 GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany Prostate cancer is the predominant cancer in men in developed countries. Carbon ion therapy could be an efficient method for treating this indication due to its high conformity and radiobiological effectiveness. However, the high conformity in particle therapy is challenged by organ motion because density variations in the beam path have a larger influence on the target dose as compared to photons. The tendency to apply a hypofractionated prostate treatment protocol emphasizes this problem. In photon therapy two approaches of pre-treatment registration are applied: bony anatomy or soft tissue registration, but the impact of the registration method on the quality of photon treatment is low (Kalz et al. 2008). In particle therapy the selection of bony anatomy pre-treatment registration seems to be more appropriate for treatment because density variations in the beam path appear not to have a significant impact on dose distribution. However, because of interfractional target motion additional extension of the clinical target volume is needed to ensure homogeneous target coverage (Rietzel, Bert, 2010). Although the application of soft tissue registration may mitigate impact of the target motion on the dose distribution, anatomy variations in the beam path may cause relevant dose shifts in beam direction. In this study we analyze treatment plan robustness concerning interfractional target motion, selection of registration method, motion induced density variations in the beam path and potential rectal toxicity. The aim of this treatment planning study is to develop a target volume definition method which provides the greatest treatment plan robustness. In the presented study Megavoltage Computed Tomography (MVCT) data acquired on a daily basis from a TomoTherapy unit from the Radiation Oncology Department at the Heidelberg University Clinic were used. For dose optimization and analysis of recalculated dose distributions all MVCT images were entirely contoured. The clinical target volume was defined according to world-wide commissioned segmentation protocol, as the volume of prostate extended by a six millimetres isotropic margin and including seminal vesicles. The treatment plans were optimized on selected MVCT images, separately for bony anatomy and soft tissue registration method. Independent treatment plans were optimized for the different target volume definition methods. The optimization and recalculation was performed with the TRiP Package (Treatment Planning for Particles, Krämer et al. 2000) successfully used in the Carbon Ion Therapy Pilot Project hosted at GSI, Darmstadt, Germany. Treatment plan recalculations performed on daily MVCT images contain information about the impact of target motion on dose distribution and indicate on anatomical variations. In order to perform assessment of recalculated therapy plans we analyzed: