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Variable RBE Models Predict Possible Underestimation of Vaginal Dose for Anal Cancer Patients Treated Using Single-Field Proton Treatments

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A McNamara

A McNamara*, T Underwood , J Wo , H Paganetti , Massachusetts General Hospital & Harvard Medical School, Boston, MA


SU-F-T-132 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Anal cancer patients treated using a posterior proton beam may be at risk of vaginal wall injury due to the increased linear energy transfer (LET) and relative biological effectiveness (RBE) at the beam distal edge. We investigate the vaginal dose received.

Five patients treated for anal cancer with proton pencil beam scanning were considered, all treated to a prescription dose of 54 Gy(RBE) over 28-30 fractions. Dose and LET distributions were calculated using the Monte Carlo simulation toolkit TOPAS. In addition to the standard assumption of a fixed RBE of 1.1, variable RBE was considered via the application of published models. Dose volume histograms (DVHs) were extracted for the planning treatment volume (PTV) and vagina, the latter being used to calculate the vaginal normal tissue complication probability (NTCP).

Compared to the assumption of a fixed RBE of 1.1, the variable RBE model predicts a dose increase of approximately 3.3 ± 1.7 Gy at the end of beam range. NTCP parameters for the vagina are incomplete in the current literature, however, inferring value ranges from the existing data we use D₅₀ = 50 Gy and LKB model parameters a=1-2 and m=0.2-0.4. We estimate the NTCP for the vagina to be 37-48% and 42-47% for the fixed and variable RBE cases, respectively. Additionally, a difference in the dose distribution was observed between the analytical calculation and Monte Carlo methods. We find that the target dose is overestimated on average by approximately 1-2%.

For patients treated with posterior beams, the vaginal wall may coincide with the distal end of the proton beam and may receive a substantial increase in dose if variable RBE models are applied compared to using the current clinical standard of RBE equal to 1.1. This could potentially lead to underestimating toxicities when treating with protons.

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