Journal
BRACHYTHERAPY
Volume 18, Issue 5, Pages 701-710Publisher
ELSEVIER SCIENCE INC
DOI: 10.1016/j.brachy.2019.04.005
Keywords
High-dose-rate brachytherapy; High-dose-rate brachytherapy monotherapy; Prostate cancer fractionation schemes; Intrafraction repair; Dose protraction; Biological effective dose calculations
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PURPOSE: This work quantifies the influence of intrafraction DNA damage repair and cellular repopulation on biologically effective dose (BED) in Ir-192 high-dose-rate brachytherapy for prostate cancer. In addition, it examines the effect of source-decay-induced BED variation for patients treated at different time points in a source exchange cycle. MATERIALS AND METHODS: Current fractionation schemes are based on simplified-form BED = nd(1 + d/(alpha/beta)), which assumes that intrafraction repair, interfraction repair, and repopulation are negligible. We took accepted radiobiological parameters of T-k, T-p, and alpha from the recommendations of the AAPM TG-137, and recalculated the full-form BED. Fraction times were normalized to require 15 min for 20 Gy at 10 Ci. Calculations were carried out for both alpha/beta = 1.5 and 3 Gy. RESULTS: After accounting for intrafraction repair, interfraction repair, and/or repopulation, full-form BED calculations showed significant values, as compared with simplified-form BED. For 1-fraction 20 Gy fractionation, the full-form BED was only 64-82% of the simplified-form BED. Dose protraction effects were milder for smaller prescriptions (6 Gy/Fx), where full form was 87-94%. With regard to source decay, BED varied >20% for patients treated at the beginning and the end of a source exchange cycle for 20 Gy single-fraction prescription. CONCLUSIONS: Repair and repopulation can be significant in monotherapy high-dose-rate for prostate cancer. As fractionation schemes are established, the simplified BED calculation may not be appropriate. Investigators should consider evaluating BED as a range rather than a discrete value when presenting results unless source activity is explicitly incorporated as well. (C) 2019 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.
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