Dosimetric Predictors of Toxicity in Patients With Early-Stage Prostate Cancer Treated With Moderately Hypofractionated IMRT or PBT: A Multi-Institutional Analysis

Abstract

<h3>Purpose/Objective(s)</h3> Limited published data address modifiable risk factors for significant toxicity complicating moderately hypofractionated schedules of intensity-modulated radiation therapy (IMRT) and proton-beam therapy (PBT) in patients with early-stage prostate cancer (PC). This study explored dosimetric predictors of Grade 3+ gastrointestinal (GI) and genitourinary (GU) toxicity following IMRT or PBT in this patient population. <h3>Materials/Methods</h3> The study population included patients from five tertiary referral centers treated from 2002-2018 for low- or intermediate-risk biopsy-proven prostate adenocarcinoma. All patients were treated with moderately hypofractionated radiation defined as 250–300 cGY per daily fraction given over 4-6 weeks. Primary outcomes were late GI and GU toxicity as defined by CTCAE v4.0. Dosimetric variables included PTV max dose, PTV volume, bladder V31 Gy, bladder V50 Gy, bladder V70 Gy, bladder max dose, rectum V31 Gy, rectum V50 Gy, rectum V70 Gy, rectum max dose, penile bulb V10 Gy, penile bulb V15 Gy, and the left and right femoral head V40 Gy. The association of dosimetric variables with treatment toxicity was assessed with the Wilcoxon signed-rank test. <h3>Results</h3> Dosimetric data were obtained for 1139 patients, including 667 treated with IMRT and 472 treated with PBT. Median follow-up for the two groups was 46.0 and 48.5 months, respectively. Late toxicity rates for the total population were 4.7% for Grade 3+ GU toxicity and 1.5% for Grade 3+ GI toxicity. Grade 3+ GU toxicity was associated with prostate volume, bladder V31 Gy and bladder V50 Gy (<i>P</i> = 0.016, <i>P</i> = 0.018, <i>P</i> = 0.028), but not with bladder V70 Gy or maximum dose to the bladder. Grade 3+ GI toxicity was not associated with any of the dosimetric variables. When analyzed by treatment group, Grade 3+ GU toxicity was observed in 3% of the IMRT cohort and 1.7% of the PBT cohort. Grade 3+ GI toxicity was seen only in PBT and was observed in 1.5% of patients. Proton plans were associated with less radiation exposure of rectum and bladder at lower doses (rectum V31 Gy: 13% vs 16%, <i>P</i> < 0.001; bladder V31 Gy: 18% vs 26%, <i>P</i> < 0.001), while IMRT was associated with less radiation exposure of rectum and bladder at higher doses (rectum V70 Gy: 0.4% vs 2%, <i>P</i> < 0.001; bladder V70 Gy: 1% vs 3%, <i>P</i> < 0.001). Photon plans yielded superior V40 Gy to the femoral heads, and superior V10 Gy and V15 Gy to the penile bulb (<i>P</i> < 0.001, <i>P</i> < 0.003, <i>P</i> < 0.002). <h3>Conclusion</h3> In this multi-institutional analysis of 1139 early-stage PC patients treated with moderately hypofractionated IMRT or PBT, risk of serious late GU and GI complications was low for both treatment groups. Analysis of the dosimetric variables identified an association of Grade 3+ GU toxicity with low-dose radiation exposure of the bladder. Grade 3+ GI toxicity was unrelated to dosimetric variables. PBT plans yielded less exposure of non-target organs to lower-dose radiation while IMRT plans yielded less exposure of non-target organs to higher-dose radiation.