Radiation for Anorectal Cancers in Patients With a History of Prostate Radiation Therapy

Abstract

RT for rectal and anal cancer after prostate cancer RT is usually avoided due to fear of complications, and data on this topic is scarce. Our aim was to evaluate outcomes and toxicity associated with RT in this group of patients.We conducted a single institution retrospective study of patients treated with RT for rectal or anal cancer after prior prostate RT from 1/1/1995 - 8/1/2019. Acute and long-term toxicities were collected. Treatment plans were extracted to assess doses to organs at risk and target coverage. Wilcoxon rank sum test was used for correlation between dosimetry and toxicities. Estimated cumulative incidence was used for local progression.We identified 29 patients who received RT after prostate cancer RT, 19 with rectal cancer and 10 with anal cancer. None of those patients had metastatic disease. Prior prostate RT was delivered using low dose rate brachytherapy (LDR) in 15/29 patients with a median dose of 145 Gy (144-160), external beam RT (EBRT) in 10 patients with 79.2Gy/44 or 81Gy/45 fractions (fr), and EBRT + LDR in 4 patients with 50.4/28 + 80-110 Gy I-125 or Pd-103 LDR. RT for rectal cancer was delivered most commonly with 50.4Gy/28 fr or 1.5 Gy twice daily fractionation to 30-45 Gy (53% of patients). The most commonly used RT dose for anal cancer was 50Gy/25 fr. Brachytherapy was used in 6 (21%) patients: as a boost after EBRT in 2 patients with anal cancer and alone in 4 rectal cancer patients. Median interval between prostate and anorectal RT was 134 months (6-303). 68% and 80% of rectal and anal cancer patients got concurrent chemotherapy. Table 1 summarizes toxicities associated with anorectal re-RT. Two patients developed fistulas, one was urinary cutaneous after prostate LDR and 45Gy/25fr for rectal cancer, and the other recto vesicular after prostate LDR and 50Gy/25fr for anal cancer; both repaired by extensive surgery. Both patients had more than 10 years between the two RT courses. In 21 patients with available dosimetry, coverage was adequate with a GTV V100% of 98% for rectal cancer patients and 96% for anal cancer patients. There was no significant correlation between toxicities and dosimetric values probably due to the small sample size. The estimated 5-year local progression incidence for all patients was 22%: 11% for rectal cancer, and 4% for anal cancer patients.RT of anorectal cancer after prior prostate cancer RT is feasible but poses a risk of fistulae. Further studies, possibly using proton therapy and/or rectal hydrogel spacers, are needed to further establish the feasibility of RT in this context, decrease toxicity, and improve outcomes.