Safety of carbon-ion radiotherapy for prostate cancer after rectal surgery: a single-institution retrospective study.

Carbon-ion radiotherapy (C-ion RT) is effective in treating prostate cancer. However, the safety of this therapy in patients with a history of rectal surgery remains unclear. This study aimed to confirm the feasibility and efficacy of C-ion RT in patients with prostate cancer after rectal surgery. This study comprised 33 consecutive patients with prostate cancer who underwent C-ion RT at our institute from October 1995 to May 2024 with a history of rectal surgery for their colorectal cancers. The prescribed doses were 63.0 Gy [relative biological effectiveness (RBE)] in 20 fractions (fr), 57.6 Gy (RBE) /16 fr, and 51.6 Gy (RBE) or 54.0 Gy (RBE) /12 fr. The Common Terminology Criteria for Adverse Events, version 5.0, was used to assess toxicity. Risk factors for rectal bleeding, including patient and treatment characteristics, and dosimetric parameters were statistically analyzed. The median patient age and observation period were 70 years and 53.1 months, respectively. Late grade 1 and 2 gastrointestinal toxicities were observed in six (18.2%) and one (3.0%) patients, respectively. Late grade 1 gastrointestinal toxicities included rectal bleeding, and late grade 2 gastrointestinal toxicities included faecal incontinence. The development of late rectal bleeding was significantly associated with the duration from surgery to radiation (P = 0.040), especially <5 years (P = 0.011), and rectal Dmean (P = 0.018). C-ion RT is safe for patients with prostate cancer after rectal surgery, and its toxicity is manageable. A shorter time to radiation from rectal surgery, especially <5 years, and rectal Dmean are risk factors for late rectal bleeding.