3183 - Toxicity Comparison of Whole Pelvic vs. Prostate Only Proton Therapy - Results from the Proton Collaborative Group PCG 001-09

A. Raghavan¹, J. H. C. Chang², S. A. Mihalcik³, H. Tsai⁴, L. R. Rosen⁵, J. Zeng⁶, M. V. Mishra⁷, C. E. Vargas⁸, N. Sebastian⁹, C. J. Rossi¹⁰, A. M. Chhabra¹, I. J. Choi¹, C. B. Simone II¹, and S. Hasan¹; ¹New York Proton Center, New York, NY, ²Oklahoma Proton Center, Oklahoma City, OK, ³Northwestern Medicine Proton Center, Warrenville, IL, ⁴ProCure Proton Therapy Center, Somerset, NJ, ⁵Willis-Knighton Proton Therapy Center, Shreveport, LA, ⁶Department of Radiation Oncology, University of Washington - Fred Hutchinson Cancer Center, Seattle, WA, ⁷Maryland Proton Treatment Center, Baltimore, MD, ⁸Department of Radiation Oncology, Mayo Clinic, Phoenix, AZ, ⁹Emory Proton Therapy Center, Atlanta, OH, ¹⁰California Protons Cancer Therapy Center, San Diego, CA

Purpose/Objective(s): Whole pelvis (WP) radiation with photons is associated with higher toxicity in prostate cancer patients compared to prostate radiation alone. Such differential toxicity has not been defined with proton therapy (PT). We hypothesize PT can mitigate a significant differential in toxicity between WP and prostate only irradiation.

Materials/

Methods: Using the Proton Collaborative Group prospective registry (NCT01255748), we compared the acute and late gastrointestinal and genitourinary toxicity of proton therapy (PT) delivered to the prostate only versus WP PT. Parameters that predicted for adverse events (AEs) were assessed using univariable and multivariable binomial regression analysis.

Results: A total of 6473 patients with prostate cancer were included with an enrollment period from 7/2009 to 9/2023 and a median follow-up of 21 months. 874 patients were treated with WP PT while 5599 received radiation to the prostate alone. The median baseline International Prostate Symptom Score (IPSS) was 7 and the median age was 68 (IQR 64-73). Patients less likely to receive WP PT include those with higher BMI (OR = 0.98; p < 0.001), hypofractionated treatments (OR: 0.72; p = 0.01), and rectal balloon placement (OR: 0.21; p < 0.001).

Acute grade 2 GU AEs occurred in 22.1% of patients when treated with prostate-only PT compared to 27.5% with WP PT (OR: 1.36; CI 1.12-1.64; p = 0.001). Conversely, 11.7% of WP-PT patients developed late GU grade 2 AEs compared to 18.1% in prostate-only patients (OR: 0.6; CI: 0.47-0.77; p < 0.001). Among patients who received prostate-only PT, 5.1% had late grade 2 GI AEs compared to 3.1% with WP PT (OR: 0.61; CI: 0.39-0.96; p = 0.032).

The rate of grade 3 GU AEs were 0.6% and 0.8% in prostate only and WP PT cohorts, respectively. The rate of grade 3 GI AEs were 0.5% and 1.0% in prostate only and WP PT cohorts, respectively. There were no statistical differences between cohorts for grade 3 AEs.

There was no difference in any AE parameter between WP and prostate-only PT with multivariable regression. Lower IPSS score correlated with reduced toxicity in both acute grade 2 (OR: 0.048; CI: 1.00-1.07; p < 0.001) and late grade 2 GU conditions (OR: 0.0497; CI: 1.04-1.06; p < 0.001). Rectal balloon placement was associated with acute grade 2 GU (OR: 2.14; CI: 1.65-2.78; p < 0.001), late grade 2 GU (OR: 2.25; CI: 1.65-3.08; p < 0.001), and late grade 2 GI (OR: 3.09; CI: 1.66-5.78; p < 0.001) AE conditions.

Conclusion: After adjustment for confounding variables, there appears to be no additional toxicity risk associated with WP compared to prostate only radiation with PT. Further evaluation is warranted to confirm these findings.