Program Information

Effect of Variable RBE On the Distal Range of the Passive Scattering Proton Beams

N Sahoo

N Khuntia , A Gautam , X Zhang , X Zhu , M Gillin , N Sahoo*, MD Anderson Cancer Ctr., Houston, TX

Presentations

SU-I-GPD-T-107 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To determine the amount of increase in the distal range of the passive scattering proton beams due to variable RBE and to devise a strategy to reduce the RBE dose to the organs at risk (OAR) located in the distal dose fall off region of the spread out Bragg peak (SOBP)

Methods: An empirical variable RBE model was used to covert the measured depth dose curves of the SOBPs of the passive scattering proton beams to biological effective dose (BED = dose x RBE). The changes in the beam range (the depth of the distal 95% dose) and the longitudinal penumbra width (the distance between distal 95% and distal 10% dose) with variable RBE were determined relative to those with fixed RBE of 1.1.

Results: The increase in the distal range of the proton beams due to a variable RBE was found to be energy dependent, and was 4 to 1 mm for the beams with energy of 250 MeV and 100 MeV respectively relative to the fixed RBE. The dose fall off with variable RBE was found to be affected more for the high energy beams compared to low energy beams with the longitudinal penumbra width being slightly smaller (2mm for 250 and 225 MeV beams) compared to fixed RBE.

Conclusion: Variable RBE in the distal dose fall off region increases the distal range of the beam and needs to be considered in the treatment planning while deciding the distal margins for the targets in passive scattering proton beam plans. A reduced distal margin around the target can be used to offset the observed increase in the distal range to reduce the dose to the OAR located near the distal dose fall off region without affecting robustness of target coverage with the prescribed BED.


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