H. S. McGinnis, D. Bohannon, Y. Wang, A. J. Zafar, Z. Diamond, J. Zhou, and S. A. Patel; Department of Radiation Oncology, Emory University, Atlanta, GA
Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) use for localized prostate cancer (PC) is rising. Proton SBRT may become more widely adopted given patient convenience, cost-effectiveness, and favorable published outcomes. Genitourinary (GU) toxicity is common, and predictors of adverse GU outcomes are needed. Given the steep dose gradient and lateral penumbra associated with proton therapy, we hypothesize that bladder neck dosimetry, rather than overall bladder volume, is a better predictor of adverse GU toxicity. Additionally, due to end-of-range uncertainty, interfractional set-up variations on delivered dose to organs at risk (OAR) may be more significant with proton SBRT. Herein, we evaluated the impact of bladder neck versus whole bladder dosimetry on GU toxicity in a modern cohort of men treated with proton SBRT. Additionally, we compare interfractional variation in whole bladder versus bladder neck in this cohort. Materials/
Methods: Between 2020-2022, 43 consecutive patients with low or intermediate risk PC were treated with proton SBRT (36.25-40 Gy in 5 fractions). Whole bladder dosimetry was used for clinical planning directives; in this post-hoc study, bladder neck/trigone was retrospectively contoured on treatment planning CT (TPCT) and each of pre-treatment cone beam CTs (CBCT). Dosimetry data was calculated from each image set. GU toxicity was assessed by International Prostate Symptom Score (IPSS) and Expanded Prostate Cancer Index Composite (EPIC-CP, urinary domain) surveys at 3-6 month intervals for 12 months following treatment. Minimal clinically important decline (MCID) in GU quality-of-life (QOL) was defined as the difference of one-half standard deviation from baseline scores. Independent Mann-Whitney U Test was used for the volume and dosimetry data analysis. Results: In this cohort, 8 men (18.6%) experienced a MCID in GU QOL within 12 months of proton SBRT. Bladder neck V37 (p=0.05) and V36.25 (p=0.037) were significantly associated with incidence of GU decline. No whole bladder dosimetric parameters were associated with post-treatment GU QOL decline. With respect to OAR volume assessment, the mean TPCT volume of the bladder neck and whole bladder volume was 2.75 cc (sd 1.47 cc) and 310.6 cc (sd 149.5 cc), respectively. The mean change in volume from TPCT to CBCTs for bladder neck and whole bladder was -0.04 cc (sd 0.36 cc) and +17.5 cc (sd 104.2 cc), respectively. Conclusion: For men with PC treated with proton SBRT, bladder neck dosimetry is associated with post-treatment GU toxicity, which may not be predicted by conventional whole bladder dosimetry. Further, there is minimal interfractional variation of bladder neck, compared with whole bladder volume. The bladder neck may be a more reliable OAR for prostate proton SBRT planning.
