MO-A-213AB-09: Hypofractionated Proton Therapy of the Prostate: The Impact of the Uncertainties in Dose Delivery and Alpha/Beta Ratio on Tumor Dose Escalation

Medical Physics
Yi Wang, Alexei Trofimov

Abstract

Hypofractionation is expected to improve therapeutic ratio for prostate radiotherapy, due to the relatively low alpha/beta ratio of the prostate tumor (∼1.2 to 2.0 Gy). However, the gain in tumor equivalent dose in 2-Gy fractions (EQD2) is accompanied by the increased uncertainty in delivered dose due to inter-fractional variations. The purpose of this study is to evaluate how this trade-off is affected by the uncertainty of the tumor alpha/beta. We used serial CT images acquired from two prostate cancer patients. Target and normal organs were contoured on the simulation and daily images. A 3D conformal proton plan was designed based on standard fractionation (78 Gy in 39 fractions) and renormalized for hypofractionation (between 5 and 28 fractions). The fraction size of the hypofractionated protocols was adjusted so as to maintain the maximum rectal dose at 78 Gy-EQD2 (alpha/beta = 3 Gy). The fractional dose, calculated on each daily CT, was mapped to the simulation geometry via deformable registration. The worst-case-scenario PTV dose for a hypofractionated protocol was estimated by summing the fractions (e.g., 28) with the lowest D97%. The target dose (e.g., D100%) was evaluated for alpha/beta of 1.2 to 2.0 Gy. The dose deli...Continue Reading

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