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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
Author(s) -
Wang Yi,
Trofimov Alexei
Publication year - 2012
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4735761
Subject(s) - nuclear medicine , dosimetry , prostate cancer , prostate , medicine , proton therapy , alpha (finance) , radiation therapy , beta (programming language) , dose fractionation , cancer , radiology , computer science , surgery , construct validity , patient satisfaction , programming language
Purpose: 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. Methods: 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. Results: The dose delivery uncertainty due to inter‐fractional motion increased as the treatment became more hypofractionated. D100% was<78 Gy‐EQD2 for protocols with 28, 26, 23 and 20 fractions when alpha/beta was >1.25, 1.46, 1.68 and 1.84 Gy, respectively. At alpha/beta of 2 Gy (1.2 Gy), D99% ranged from ∼79 (81) to 85 (98) Gy‐EQD2 for treatments in 28 to 5 fractions. Below D97%, the target dose was predominantly determined by alpha/beta, and the motion impact was minimal. Conclusions: In prostate treatments, the impact of inter‐fractional motion on tumor dose escalation is small for alpha/beta <2.0 Gy, and is of minimal concern to hypofractionated proton therapy. This study was supported by the Federal Share of program income earned by Massachusetts General Hospital on C06‐CA059267, Proton Therapy Research and Treatment Center.