Program Information
How Does Beam Dose Modulation in Proton Therapy Affect RBE-Weighted Dose in Healthy Tissues?
D Giantsoudi*, J Adams , S MacDonald , H Paganetti , Massachusetts General Hospital, Boston, MA
Presentations
SU-K-108-9 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 108
Purpose: To investigate the difference in radiobiological effect between passive scattering (PS) and pencil beam scanning (PBS) proton radiation therapy techniques, due to differences in linear energy transfer (LET) distribution.
Methods: Six patients were selected, previously treated for ependymoma with PS proton irradiation in our institution. A PBS intensity modulated proton therapy (IMPT) plan was created for each patient, using the original planning computed tomography image sets and beam geometry, with average beam spot size of 3mm (sigma in air at isocenter) and original plan’s dose-volume values as optimization constraints. Three-dimensional dose and LET distributions were calculated by Monte Carlo (MC) using TOPAS both for the original PS and new PBS treatment plans. The MC dose distribution for every plan was normalized to the same target volume receiving 100% of the prescription dose (V100) as the PS plan. Variable RBE-weighted dose distributions were calculated for the normalized MC dose distributions, using a linear dose and LET-dependent model. Dose comparisons were performed in terms of target coverage and brainstem sparing for both constant and variable RBE.
Results: Regarding physical dose, better conformity was achieved with PBS planning compared to PS: increased dose coverage for the target (0.5-7.3% increase in D95) and decreased dose to the brainstem adjacent to it (0.5-2.1% decrease in maximum dose). Accounting for LET distributions and variable RBE, however, the maximum brainstem dose for the PBS plans was 1-4% higher compared to PS, due to the elevated LET values for the part of the brainstem receiving significant dose.
Conclusion: Transitioning from passive scattering to pencil beam scanning proton radiation treatment can be dosimetrically beneficial for ependymoma patients. However caution should be exercised so that the dosimetric advantage of the increased dose conformity is not negated by RBE differences. LET-guidance or LET-based inverse optimization should be applied for PBS.
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