Predictive Factors of Rectal Toxicity After Permanent iodine-125 Seed Implantation: Prospective Cohort Study in 2339 Patients

Purpose: To evaluate the incidence and the associated factors of rectal toxicity in patients with prostate cancer undergoing permanent seed implantation (PI) with or without external beam radiation therapy (EBRT) in a nationwide prospective cohort study in Japan (J-POPS) during the first 2 years.
Methods and materials: A total of 2,339 subjects were available for the analyses. Rectal toxicity was evaluated using the National Cancer Institute Common Terminology Criteria for Adverse Events version 3.0.
Results: The 3-year cumulative incidence for grade ≥2 rectal toxicity was 2.88%, 1.76%, and 6.53% in all subjects, PI group and EBRT combination therapy group, respectively. On multivariate analysis, among all subjects, grade ≥2 rectal toxicity was associated with rectal volumes receiving 100% of the prescribed dose (R100; p < 0.0001) and EBRT combination therapy (p = 0.0066). R100 in the PI group (p = 0.0254), and R100 (p = 0.0011) and interactive planning (p = 0.0267) in the EBRT combination therapy group were also associated with grade ≥2 toxicity. The 3-year cumulative incidence of grade ≥2 rectal toxicity was 3.80% and 1.37% for R100 ≥ 1 mL and R100 < 1 mL, respectively, in the PI group (p = 0.0068), and 14.09% and 5.52% for R100 ≥ 1 mL and R100 < 1 mL, respectively, in the EBRT combination therapy group (p = 0.0070).
Conclusions: Rectal toxicity was relatively rare in this study compared with previous reports. For Japanese prostate cancer patients, R100 < 1 mL in both PI and EBRT combination therapy groups and interactive planning in EBRT combination therapy group may be effective in decreasing the incidence of rectal toxicity.