Rectum Dose-Volume Histogram (DVH) Parameters And Patient-Reported EPIC-Bowel Domain For Patients Undergoing Radiotherapy (RT) For Prostate Cancer (PC): A Single-Center Prospective Registry Experience

Abstract

Prostate cancer therapy toxicities can affect QOL. We report one of the first analyses of rectal dose and changes in QOL measured with the EPIC bowel domain in PC patients treated with curative intent RT in a large, single-institution prospective registry. Two hundred fifty-two pts with localized PC were treated with curative intent RT from 2016- 2018 on an IRB approved prospective registry. The Expanded Prostate Cancer Index Composite (EPIC) health-related QOL instrument was administered at baseline, end-of-treatment, 3, 6, 12 months (m), and then annually. A higher EPIC score implies less bowel symptom burden. DVH parameters for the rectal structure were computed, Dx was the dose (Gy) to a % of the rectum and Vx was the % volume receiving a given dose (Gy). Rectal dose was analyzed using BED (alpha/beta = 3), EQD2Gy, and total dose. In order to determine optimal cut-points to find clinically meaningful DVH, we utilized logistic regression and defined the ‘best’ cut-point to be the value that maximized Youden’s Index. Repeated measures mixed models were then implemented to determine the effect of patient, clinical, and treatment factors (including DVH) on patient-reported bowel symptom burden (EPIC-Bowel). Median follow-up was 20mo(range:2.5–40), pre-RT PSA = 6.12(1.3–129), T Stage T1c(T1b–T3b), Gleason Score 7(range 6-10), and 99% N0. The median prescribed dose was 70Gy (38–79.2) in 5 to 44 fractions. Standard fractionation (1.8-2Gy) was used in 120 pts (48.2%), moderate hypofractionation (2.5-3Gy) in 87 pts (35.1%), and extreme hypofractionation (>6Gy) in 42 pts (16.7%). Globally, treatment had minimal effect over EPIC-bowel scores which remained unchanged over time, with a median baseline = (96.4), end-of-treatment = (94.6), 3mo = (95.4), 6mo = (96.4), 12m = (94.6), and 24m = (96.4). However, rectal dose affected QOL with the strongest DVH signals noted for BED V51Gy and EQD2 V22Gy—with an optimal cut-point of 14.4% and 16.4%, respectively. On multivariate modeling BED V51Gy >14.4% was significantly associated with a decline of the epic bowel domain of 2.1 points (p = 0.01), and an EQD2 V22Gy >16.4% was significantly associated with a decline of 2.3 points (p<0.003). Rectal dose, specifically BED V51Gy>14.3% and EQD2 V22Gy>16.4%, are significantly associated with decline in bowel related QOL in patients undergoing definitive RT for localized PC. This is one of the first studies to examine rectal dose and correlate them to patient reported treatment related QOL.