EQD2 of HDR Brachytherapy for Vaginal Cancer Based On a Novel Model
B Wang1*, S Li1, B Micaily1, A Hollander2, T Giaddui1, C Miyamoto1, (1) Dept. of Rad. Onc.,Temple University Hospital, Philadelphia, PA, (2) Holy Redeemer Hospital, Meadowbrook, PA
Presentations
PO-GePV-T-442 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: The α/β ratio of the linear-quadratic (LQ) model is obtained by fitting the cell-survival curves (CSC) in the low-dose domain. Applying the same α/β ratio might be inappropriate to predict the biological effects for treatments with large fractional dose. Recently a unified multi-activation (UMA) model has been proposed to fit CSC over the entire dose range. Based on the UMA model, we calculate EQD2 and compare with LQ-EQD2.
Methods: Fifteen HDR cases treated with multi-channel vaginal cylinders are selected for this study. D90 (minimum dose received by 90% or less of the volume) of the high-risk CTV (HRCTV) with and without applicator subtraction are collected from the treatment plans for each fraction. EQD2 of the HRCTV and HRCTV-applicator D90 are calculated based on the LQ model (assuming α/β=10Gy) and the UMA model with fitting parameters corresponding to two cell-lines, D0=2.71Gy, n=2.8 (NHIK3025 with 40Gy pre-irradiation to medium, “UMA-1”), and D0=1.11Gy, n= 1.5 (RL95-2, “UMA-2”).
Results: For 10 cases with 7Gy/fraction prescription, the UMA-1 and UMA-2 EQD2 cumulative D90 doses are different from the LQ EQD2 on average by 2.47% and -21.25% for HRCTV, and by 3.06% and -18.76% for HRCTV-applicator, respectively. For 5 cases with 6Gy/fraction, the differences are 3.39% and -16.82% for HRCTV, and 3.71% and -14.77% for HRCTV-applicator, respectively. We also note that excluding the HDR applicator or not from HRCTV could be significant for the evaluation of biological effects of radiation treatment. For example, the LQ EQD2 cumulative doses of HRCTV-applicator are different from those of HRCTV by -11.26% on average(-8.36% to -16.67%).
Conclusion: Using a single α/β ratio obtained from low-dose domain of CSC for HDR EQD2 calculation could be overly simplistic. The novel UMA-model fits CSC over entire dose range and provides a new way to calculate EQD2, which may be more accurate.
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