3377 - Late Proctitis Rates Following Moderately-Hypofractionated Image-Guided Prostate Radiotherapy Using Fiducials vs. Cone Beam Computed Tomography

Purpose/Objective(s):

Radiation proctitis is a principal dose-limiting toxicity of prostate radiotherapy (RT). Utilization of image-guided radiation therapy (IGRT) improves the therapeutic ratio by allowing for more precise delivery of RT to the prostate while avoiding surrounding organs at risk (OARs) such as the rectum. The purpose of this study is to compare late proctitis rates following prostate RT between two image-guidance methods, implanted fiducials vs. cone beam computed tomography (CBCT).

Materials/Methods:

We performed a retrospective analysis of 445 patients treated with definitive prostate RT with volumetric modulated arc therapy to a dose of 70 Gy in 2.5 Gy fractions with fiducials (n=206) or CBCT (n=239) between 10/2020 and 10/2022 within an integrated healthcare system. Patients who received IGRT with both fiducials and CBCT were excluded, as were those who underwent pelvic RT. Late proctitis was graded according to the Common Terminology Criteria for Adverse Events v5.0. Univariable (UVA) and multivariable (MVA) logistic regression analyses were performed to identify predictors of late grade 2+ (G2+) proctitis.

Results:

Median follow-up was 32.5 months. Median patient age was 70 years; 9% had low-risk, 62% had intermediate-risk, and 29% had high-risk disease. Median clinical target volume was similar between the two groups (58cc for fiducials vs. 62cc for CBCT, p=0.12). Planning target volume (PTV) was smaller in the fiducial group (137cc vs. 156cc, p=0.02). There were no differences in median rectal V45 (19% vs. 20%, p=0.13), V55 (13% vs. 14%, p=0.11), or V65 (7.4% vs. 8.2%, p=0.10) between the fiducial and CBCT groups, respectively. Among the entire cohort, 16% experienced G2 and 2% had G3 proctitis. No G4-5 toxicities occurred. On UVA, clinical factors associated with G2+ proctitis included PTV volume (HR=1.01, p=0.04) and use of CBCT vs. fiducials (22% vs. 12%) (HR=2.06, p=0.006). The strongest dosimetric predictor for G2+ proctitis was rectal V65 (HR=1.12, p<0.001). On MVA, V65 >8% (HR=2.30, p=0.002) and CBCT (HR=1.95, p=0.01) were associated with increased rates of G2+ proctitis, while PTV volume (HR=1.00, p=0.22) was not. On MVA, in the subgroup of patients treated with CBCT, V65 >8% remained associated with increased rates of G2+ proctitis (30% vs. 14%) (HR=2.65, p=0.004), whereas no difference was observed in the fiducial group (16% vs. 9%) (HR=1.81, p=0.18).

Conclusion:

CBCT-based prostate RT was associated with an increased risk of late G2+ proctitis when compared to implanted fiducials; however, this risk was reduced in patients with rectal V65 <8%. Employment of this more conservative dosimetric parameter may mitigate the risk of proctitis in patients treated with CBCT alone.