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Feasibility of Intensity Modulated Proton Therapy for Base of Skull Chordomas
S742
International Journal of Radiation Oncology Biology Physics
The approach is also applicable to other treatment modalities (e.g., x-rays or brachytherapy). Author Disclosure: D. Mah: None. C. Chen: None. O. Cahlon: None. H. Tsai: None. E. Hug: None. W. Hsi: None. M. Fagundes: None. B. Chon: None.
modulation wheels were considered. The resulting depth dose distributions were calculated using the Monte Carlo code Geant4. Finally, the sum of beam current units delivered in one full rotation was used to predict the overhead in treatment delivery time. Results: Suitable SOBPs were obtained in all cases, with the maximum range shift being limited only by the maximum thickness of the wheels. However, the gain of not having to use beam compensation or energy switching was achieved at the expense of the distal dose fall off (80% to 20%) of the shifted peaks being broadened to about 1 cm, from the original 0.5 cm. The predicted overhead in delivery time remained within reasonable limits: for range shifts up to 2 cm, the delivery time would be increased by a factor of 2.5 or less, with a maximum 5-fold increase for the largest shifts, up to 9 cm. A compromise between sharper distal dose fall off and lower delivery time can be achieved and is subject to optimization. Conclusions: The production of shifted SOBPs in compensator-free proton beams, achieved by synchronizing a modulation wheel with beam current modulation, is feasible from a theoretical point of view. Moreover, the treatment delivery time and distal dose fall off could be improved by designing a specific modulation wheel for this type of modulation. Author Disclosure: D. Sanchez Parcerisa: None. C. Ainsley: None. A. Carabe: None.
3409 Dosimetric Effect of Intratreatment Target Volume Reduction in Bulky Head-and-Neck Cancer With Intensity Modulated Proton Therapy K.W. Merrell, C.J. Beltran, J.M. Miller, K.A. Hoeft, S.K. Wurgler, J.M. Kuball, M.A. Neben-Wittich, D.J. Ma, Y.I. Garces, and R.L. Foote; Mayo Clinic Rochester, Rochester, MN Purpose/Objective(s): Studies using intensity-modulated proton therapy (IMPT) have shown feasibility of PT for cancers of the head and neck. A challenge to IMPT is intra-treatment (IT) change of target volumes (TV). To assess the impact of volumetric change during treatment, we report the first 4 patients of our institutional prospective study evaluating the need for IT adaptive planning. Materials/Methods: Four patients with bulky N2b-N3 disease were selected for this study. All patients were treated with intensity modulated radiation therapy (IMRT) using photons and received 6 weekly CT scans during RT. Multiple field IMPT plans using either bolus or range shifter (RS) were created. Gross tumor volume, planning and clinical target volumes and organs at risk (OAR) were contoured. Original IMPT plans were transferred to each weekly scan and dose-volume histograms were used to measure dosimetric changes. Results: The mean decrease in tumor volume at 2 and 7 weeks was 9% and 48.1%, respectively. The mean weight loss was 14% body weight. The maximal decrease in TV coverage at 7 weeks ranged from 12% to 58.7%. The decrease was more significant in the bolus plan. The mean decrease in TV coverage in bolus compared to RS was 13.4% vs 4.4% and 32.7% vs 8.4% at 2 and 7 weeks, respectively. By week 2 OAR had significant variations in dosimetry in both plan types, such as an increase in mean esophagus dose by an average of 103.2%. Also observed at week 2 was a 15%, 12%, 31%, and 27% increase in mean dose to the larynx, spinal cord, brain stem, and parotid gland, respectively. Conclusions: IT changes in TV result in significant TV and OAR dose variation and have potential to decrease tumor control and worsen morbidity. This effect is more prominent in bolus plans compared to RS plans. Our initial data suggest the importance of weekly IT adaptive proton planning in patients with bulky head and neck cancer. Author Disclosure: K.W. Merrell: None. C.J. Beltran: None. J.M. Miller: None. K.A. Hoeft: None. S.K. Wurgler: None. J.M. Kuball: None. M.A. Neben-Wittich: None. D.J. Ma: None. Y.I. Garces: None. R.L. Foote: None.
3410 Fast Range Switching of Passively Scattered Proton Beams Using a Modulation Wheel and Dynamic Beam Current Modulation D. Sanchez Parcerisa, C. Ainsley, and A. Carabe; University of Pennsylvania, Philadelphia, PA Purpose/Objective(s): In proton radiation therapy, the range of particles in the patient body is determined by the energy of the protons. For most systems, the energy switching time is the order of a few seconds, which becomes a serious obstacle for continuous dose delivery techniques requiring adaptive range modulation, such as proton arc therapy. This work analyzes the feasibility of using the range modulation wheel, an element in the beamline used to form the spread-out Bragg peak (SOBPs), to modify the range of the beam without the use of compensators. Materials/Methods: While delivering proton beams in double scattering mode, the beam current can be synchronized with the range modulation wheel rotation by defining a current modulation pattern. For a given beam with a range of 15 cm in water, different current modulation patterns were computed to construct similar SOBPs at ranges between 6-15 cm. Various
3411 Feasibility of Intensity Modulated Proton Therapy for Base of Skull Chordomas A. Kassaee, C.M. Yeager, P. Lichtenwalner, and M. Alonso-Basanta; University Of Pennsylvania Medical Center, Philadelphia, PA Purpose/Objective(s): To investigate the efficacy of Intensity Modulated Proton Therapy (IMPT) with pencil beam scanning for base of skull chordomas in comparison to what was treated at our institution. The modalities of treated plans include intensity modulated photon therapy (IMRT), various proton techniques excluding IMPT, or a combination of these. Materials/Methods: IMPT plans were generated on a treatment planning system for six patients previously treated with a prescription dose (PX) of 79.2 Gy (RBE). All patients were on our clinical protocol and this study was approved by our internal Institutional Review Board (IRB). IMPT plans were optimized such that 95% of planned treatment volume (PTV) received 95% of the PX (D95) while aiming to achieve the normal tissue dose constraints. The OARs reviewed were brainstem, spinal cord, optic chiasm, optic nerves, and cochlea. Results: Dose volume histograms (DVH) for each plan were calculated for dose evaluations. For IMPT plans, the 95% volume of PTV received doses ranging from 61.09 to 74.15 Gy (RBE) with a mean dose of 68.76 Gy (RBE) for IMPT plans as compared to a mean dose of 65.88 Gy (RBE) for treated IMRT, DS or combination plans and a range of 62.13 Gy (RBE) to 74.19 Gy (RBE). The maximum average doses to brainstem, spinal cord, optic chiasm, right optic nerve, left optic nerve, right cochlea, and left cochlea for IMPT plans are 62.1, 42.25, 48.53, 42.48, 41.61, 57.07, and 55.95 Gy (RBE) respectively. The treated plans resulted in average maximum doses of 65.87, 46.33, 43.19, 38.76, 37.30, 60.43, and 63.17 Gy (RBE) accordingly. Conclusions: The quality of the IMPT plans were patient-dependent. No specific trend in volume size or location was found. Spot scanning IMPT plans compare favorably to plans used for treatment for a majority of patients. Difficulty did arise when the target volume abutted or deformed OARs such as the brain stem, spinal cord, or optic chiasm. In addition, the mean dose to OARs from IMPT plans were considerably lower than the treated plans for the majority of OARs. It is currently unclear if this is meaningful clinically in regards to toxicity, a topic of interest at our institution. IMPT provides an acceptable treatment alternative to full IMRT photon, mixed modality, and broad beam single field uniform dose proton plans for the treatment of base of skull chordomas. Author Disclosure: A. Kassaee: None. C.M. Yeager: None. P. Lichtenwalner: None. M. Alonso-Basanta: None.
International Journal of Radiation Oncology Biology Physics
The approach is also applicable to other treatment modalities (e.g., x-rays or brachytherapy). Author Disclosure: D. Mah: None. C. Chen: None. O. Cahlon: None. H. Tsai: None. E. Hug: None. W. Hsi: None. M. Fagundes: None. B. Chon: None.
modulation wheels were considered. The resulting depth dose distributions were calculated using the Monte Carlo code Geant4. Finally, the sum of beam current units delivered in one full rotation was used to predict the overhead in treatment delivery time. Results: Suitable SOBPs were obtained in all cases, with the maximum range shift being limited only by the maximum thickness of the wheels. However, the gain of not having to use beam compensation or energy switching was achieved at the expense of the distal dose fall off (80% to 20%) of the shifted peaks being broadened to about 1 cm, from the original 0.5 cm. The predicted overhead in delivery time remained within reasonable limits: for range shifts up to 2 cm, the delivery time would be increased by a factor of 2.5 or less, with a maximum 5-fold increase for the largest shifts, up to 9 cm. A compromise between sharper distal dose fall off and lower delivery time can be achieved and is subject to optimization. Conclusions: The production of shifted SOBPs in compensator-free proton beams, achieved by synchronizing a modulation wheel with beam current modulation, is feasible from a theoretical point of view. Moreover, the treatment delivery time and distal dose fall off could be improved by designing a specific modulation wheel for this type of modulation. Author Disclosure: D. Sanchez Parcerisa: None. C. Ainsley: None. A. Carabe: None.
3409 Dosimetric Effect of Intratreatment Target Volume Reduction in Bulky Head-and-Neck Cancer With Intensity Modulated Proton Therapy K.W. Merrell, C.J. Beltran, J.M. Miller, K.A. Hoeft, S.K. Wurgler, J.M. Kuball, M.A. Neben-Wittich, D.J. Ma, Y.I. Garces, and R.L. Foote; Mayo Clinic Rochester, Rochester, MN Purpose/Objective(s): Studies using intensity-modulated proton therapy (IMPT) have shown feasibility of PT for cancers of the head and neck. A challenge to IMPT is intra-treatment (IT) change of target volumes (TV). To assess the impact of volumetric change during treatment, we report the first 4 patients of our institutional prospective study evaluating the need for IT adaptive planning. Materials/Methods: Four patients with bulky N2b-N3 disease were selected for this study. All patients were treated with intensity modulated radiation therapy (IMRT) using photons and received 6 weekly CT scans during RT. Multiple field IMPT plans using either bolus or range shifter (RS) were created. Gross tumor volume, planning and clinical target volumes and organs at risk (OAR) were contoured. Original IMPT plans were transferred to each weekly scan and dose-volume histograms were used to measure dosimetric changes. Results: The mean decrease in tumor volume at 2 and 7 weeks was 9% and 48.1%, respectively. The mean weight loss was 14% body weight. The maximal decrease in TV coverage at 7 weeks ranged from 12% to 58.7%. The decrease was more significant in the bolus plan. The mean decrease in TV coverage in bolus compared to RS was 13.4% vs 4.4% and 32.7% vs 8.4% at 2 and 7 weeks, respectively. By week 2 OAR had significant variations in dosimetry in both plan types, such as an increase in mean esophagus dose by an average of 103.2%. Also observed at week 2 was a 15%, 12%, 31%, and 27% increase in mean dose to the larynx, spinal cord, brain stem, and parotid gland, respectively. Conclusions: IT changes in TV result in significant TV and OAR dose variation and have potential to decrease tumor control and worsen morbidity. This effect is more prominent in bolus plans compared to RS plans. Our initial data suggest the importance of weekly IT adaptive proton planning in patients with bulky head and neck cancer. Author Disclosure: K.W. Merrell: None. C.J. Beltran: None. J.M. Miller: None. K.A. Hoeft: None. S.K. Wurgler: None. J.M. Kuball: None. M.A. Neben-Wittich: None. D.J. Ma: None. Y.I. Garces: None. R.L. Foote: None.
3410 Fast Range Switching of Passively Scattered Proton Beams Using a Modulation Wheel and Dynamic Beam Current Modulation D. Sanchez Parcerisa, C. Ainsley, and A. Carabe; University of Pennsylvania, Philadelphia, PA Purpose/Objective(s): In proton radiation therapy, the range of particles in the patient body is determined by the energy of the protons. For most systems, the energy switching time is the order of a few seconds, which becomes a serious obstacle for continuous dose delivery techniques requiring adaptive range modulation, such as proton arc therapy. This work analyzes the feasibility of using the range modulation wheel, an element in the beamline used to form the spread-out Bragg peak (SOBPs), to modify the range of the beam without the use of compensators. Materials/Methods: While delivering proton beams in double scattering mode, the beam current can be synchronized with the range modulation wheel rotation by defining a current modulation pattern. For a given beam with a range of 15 cm in water, different current modulation patterns were computed to construct similar SOBPs at ranges between 6-15 cm. Various
3411 Feasibility of Intensity Modulated Proton Therapy for Base of Skull Chordomas A. Kassaee, C.M. Yeager, P. Lichtenwalner, and M. Alonso-Basanta; University Of Pennsylvania Medical Center, Philadelphia, PA Purpose/Objective(s): To investigate the efficacy of Intensity Modulated Proton Therapy (IMPT) with pencil beam scanning for base of skull chordomas in comparison to what was treated at our institution. The modalities of treated plans include intensity modulated photon therapy (IMRT), various proton techniques excluding IMPT, or a combination of these. Materials/Methods: IMPT plans were generated on a treatment planning system for six patients previously treated with a prescription dose (PX) of 79.2 Gy (RBE). All patients were on our clinical protocol and this study was approved by our internal Institutional Review Board (IRB). IMPT plans were optimized such that 95% of planned treatment volume (PTV) received 95% of the PX (D95) while aiming to achieve the normal tissue dose constraints. The OARs reviewed were brainstem, spinal cord, optic chiasm, optic nerves, and cochlea. Results: Dose volume histograms (DVH) for each plan were calculated for dose evaluations. For IMPT plans, the 95% volume of PTV received doses ranging from 61.09 to 74.15 Gy (RBE) with a mean dose of 68.76 Gy (RBE) for IMPT plans as compared to a mean dose of 65.88 Gy (RBE) for treated IMRT, DS or combination plans and a range of 62.13 Gy (RBE) to 74.19 Gy (RBE). The maximum average doses to brainstem, spinal cord, optic chiasm, right optic nerve, left optic nerve, right cochlea, and left cochlea for IMPT plans are 62.1, 42.25, 48.53, 42.48, 41.61, 57.07, and 55.95 Gy (RBE) respectively. The treated plans resulted in average maximum doses of 65.87, 46.33, 43.19, 38.76, 37.30, 60.43, and 63.17 Gy (RBE) accordingly. Conclusions: The quality of the IMPT plans were patient-dependent. No specific trend in volume size or location was found. Spot scanning IMPT plans compare favorably to plans used for treatment for a majority of patients. Difficulty did arise when the target volume abutted or deformed OARs such as the brain stem, spinal cord, or optic chiasm. In addition, the mean dose to OARs from IMPT plans were considerably lower than the treated plans for the majority of OARs. It is currently unclear if this is meaningful clinically in regards to toxicity, a topic of interest at our institution. IMPT provides an acceptable treatment alternative to full IMRT photon, mixed modality, and broad beam single field uniform dose proton plans for the treatment of base of skull chordomas. Author Disclosure: A. Kassaee: None. C.M. Yeager: None. P. Lichtenwalner: None. M. Alonso-Basanta: None.