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+/-6.4 Gy), 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.