Estimated dose to the rectum and colon in prostate cancer patients treated with exclusive radiation therapy presenting a secondary colorectal malignancy.

Abstract Aims To estimate the dose to colorectal structures after external beam radiation therapy (EBRT) delivered to prostate cancer patients who developed secondary colorectal cancers (sCRC). Materials and methods Using data from a population-based cancer registry of 1134 prostate cancer patients, 11/264 (4.2%) patients treated with EBRT presented a sCRC. To evaluate the dose delivered to the colon and rectum, each individual index patient was matched with a study case and, using the index case treatment characteristics, dose calculations were carried out on the latter. Results The median maximum, mean and minimum doses delivered to the colon or rectum affected by the sCRC were 39.3 (range 0.2–66.0), 5.4 (range 0.2–41.3) and 0.6 (range 0.2–7.8) Gy, respectively. All but three sCRCs occurred outside the treatment fields. The estimated rectal doses after prostate radiation therapy were substantially higher than those delivered to non-rectal colic structures (mean dose 47.2 ± 16.6 vs 9.4 ± 6.4Gy), but only one (9%) patient presented a rectal cancer. The differential mean doses given to the rectosigmoid junction and sigmoid colon, with or without sCRC, were not different. Conclusions These data suggest that the administered dose after EBRT for prostate cancer to the colon, excluding the rectum, may be below the Gy unit in sCRC patients.