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Inferring In-Vivo Prostate and Rectum α/β Values From Clinical Data

R Price

R Price*, C Ma , Y Dong , T Shaikh , Fox Chase Cancer Center, Philadelphia, PA


SU-I-GPD-T-637 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Hypofractionation is gaining popularity for multiple cancers. This is particularly true for the prostate which is thought to have a low α/β ratio. The purpose of this work is to infer in-vivo α/β values for the prostate and rectum, from long-term results of a phase III randomized trial.

Methods: From 2002-6, 301 men with prostate cancer were randomized to conventional IMRT (CIMRT), 76Gy in 38 fractions versus hypofractionated IMRT (HIMRT), 70.2Gy in 26 fractions. The EQD2 value for the HIMRT arm was 84.24Gy in 2.0Gy fractions assuming a prostate α/β of 1.5Gy, providing dose escalation to result in a calculated decrease in biological and/or clinical disease failure (BCDF) of 15%. An α/β of 3.0Gy yields and EQD2 of 80.03Gy. Toxicity has been analyzed to the present. Average rectal DVHs for both arms have been converted to EQD2 values using variable α/β ratios between 1Gy and 5Gy and superimposed to match the clinical toxicity outcome data.

Results: There was no significant difference in BCDF at 5 years (21.4% CIMRT; 23.3% HIMRT), indicating equivalence in dose delivered to the prostate, no dose escalation and underestimation of the prostate α/β. The prostate α/β appears to be ≥3Gy. To date there are no statistically significant differences in GI toxicity between treatment arms indicating equivalence in dose delivered to the rectum. The rectum appears to have an α/β as low as 1Gy.

Conclusion: The change in BCDF as a function of EQD2 is unknown. Our clinically-derived data support evidence that the α/β for prostate cancer, is potentially nearer 3-4Gy than the assumed 1.5Gy for the above study. When irradiating small volumes to relatively high doses, the rectum behaves like a much later responding organ as evidenced by our clinically derived low α/β of 1Gy.

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