Men who undergo radiation therapy for Prostate cancer have an increased risk of developing rectal cancer, researchers report.

“While the findings of our study do not suggest that Prostate cancer treatment should change, we recommend that the potential for developing rectal cancer be included in conversations between doctors and patients,” Dr. Nancy Baxter, from the University of Minnesota in Minneapolis, said in a statement.

Previous reports have linked prostate radiation to pelvic cancers, especially bladder cancer. However, it was unclear if such radiation increased the risk of rectal cancer.

To investigate, the researchers analyzed data from more than 85,000 men with prostate cancer who were listed in the U.S. Surveillance, Epidemiology, and End Results (SEER) registry between 1973 and 1994.

Although most of the men were treated with surgery alone, over 30,000 underwent radiotherapy. The subjects had no history of colorectal cancer and had survived at least 5 years after being treated for their prostate cancer.

During follow-up, 1437 patients developed colorectal cancer and the bulk of these malignancies occurred at the sites that were irradiated, the researchers report in the medical journal Gastroenterology.

Further analysis showed that radiotherapy increased the risk of rectal cancer by 70 percent compared with treatment with surgery alone. In contrast, treatment with radiation was not linked to cancers elsewhere in the colon “indicating that the effect is specific to directly irradiated tissue,” the team reports.

Based on these findings, “we recommend that men who have had prostate radiation should be monitored for rectal cancer starting 5 years after treatment,” Baxter noted.

In a related editorial, Dr. William M. Grady and Dr. Ken Russell, from Seattle Cancer Care Alliance, comment that the present study “highlights the fact that as more individuals become long-term survivors of their primary malignancies, clinicians will need to focus on the prevention or early detection of secondary treatment-related cancers.”

SOURCE: Gastroenterology, April 2005.