Radiation therapy for prostate cancer has been associated with an increased rate of pelvic malignancies, particularly bladder cancer.
The association between radiation therapy and colorectal cancer has not been established.
Dr Baxter and colleagues from Minnesota conducted a retrospective cohort study using Surveillance, Epidemiology, and End Results registry data from 1973 through 1994.
The research team focused on men with prostate cancer, but with no previous history of colorectal cancer, treated with either surgery or radiation who survived at least 5 years.
The team evaluated the effect of radiation on development of cancer for 3 sites, including definitely irradiated sites (rectum), potentially irradiated sites (rectosigmoid, sigmoid, and cecum), and nonirradiated sites (the rest of the colon).
The investigators used a proportional hazards model to evaluate the effect of radiation on development of colorectal cancer over time.
| Colorectal cancers developed in irradiated sites in 267 of 1437 patients|
The researchers reported that a total of 30,552 men received radiation, and 55,263 underwent surgery only.
Colorectal cancers developed in 1437 patients of which 267 had occurance in irradiated sites, 686 in potentially irradiated sites, and 484 in nonirradiated sites.
In addition, the investigative team noted that radiation was independently associated with development of cancer over time in irradiated sites but not in the remainder of the colon.
The adjusted hazards ratio for development of rectal cancer was 1.7 for the radiation group, compared with the surgery-only group.
Dr Baxter concluded, “ We noted a significant increase in development of rectal cancer after radiation for prostate cancer.”
“Radiation had no effect on development of cancer in the remainder of the colon, indicating that the effect is specific to directly irradiated tissue.”