Radiation therapy for prostate cancer poses a risk for secondary cancers

Patients who receive radiation therapy for prostate cancer may have an increased risk—albeit a low one—of developing secondary cancers of the bladder, colon, and rectum, according to a study published March 3, 2016 in The British Medical Journal.

Although the absolute risk for developing secondary cancers was low, the risk increased over time in this study. So, “for patients with a long life expectancy of 20 years or more, the possibility of secondary malignancy related to radiation needs to be included in management discussion,” the authors recommended.

Previous research has indicated that irradiation of the prostate might contribute to carcinogenesis outside the irradiated area, but studies on the risk of secondary cancers after radiation for prostate cancer have reported mixed results.

To investigate this association, researchers in Canada and the United States undertook a systematic review and meta-analysis of 21 observational studies that assessed the risk of secondary malignancies in patients who underwent radiation treatment for prostate cancer.

They found an increased risk of cancers of the bladder, colon, and rectum, but no increased risk of lung or hematologic cancers. A subgroup analysis of patients treated with radiation compared with those treated surgically confirmed this risk.

The researchers pointed out that although they found increased risks of secondary cancers, the absolute risks were low: 0.1% to 3.8% for bladder cancer; 0.3% to 4.2% for colorectal cancer; and 0.3% to 1.2% for rectal cancer.

Treatment for prostate cancer comes with higher risks of other complications, they wrote, and this information could be helpful for making decisions on such treatment. “This information could be particularly important to a large proportion of patients where treatment is recommended and … surgery or radiation would be equal options for them to choose,” said study leader Robert K. Nam, MD, MSc, Head of the Genitourinary Cancer Site Group within the Sunnybrook Health Sciences Centre at the University of Toronto, in Toronto, Canada.

The results were consistent when the researchers looked only at studies using 5- or 10-year lag periods between radiation treatment and the development of a secondary cancer. In fact, the odds ratios for bladder and rectal cancer increased with a longer lag period.

Also, the odds of secondary cancer depended on the type of radiation. Treatment with external beam radiation therapy was consistently associated with increased risk but brachytherapy was not.

“This analysis further supports the move towards even more tightly targeted external radiation techniques,” wrote Christine Eyler, MD, PhD, Clinical Fellow in Radiation Oncology at Massachusetts General Hospital, and Anthony Zietman, MD, President of the American Society for Radiation Oncology and Professor of Radiation Oncology at Harvard Medical School, in Boston, MA, in a related editorial in The BMJ.

“Perhaps most important, this study confirms our belief that second malignancy should be added to the already long list of avoidable hazards associated with treatment for those men with low-risk prostate cancer who simply need no treatment at all,” Drs. Eyler and Zietman added. “Concern about second malignancies should not, however, stand in the way of an effective and well-studied treatment being given to men with higher grade, lethal prostate cancer for whom the potential benefit simply dwarfs the risk.”