Prostate EBRT linked with risk of two other cancers

Ann Arbor, MI-Men with prostate cancer who are treated with external beam radiation therapy (EBRT) are at an increased risk for cancer of the bladder and rectum 10 years or more after their prostate cancer diagnosis, according to an analysis using data from the Surveillance, Epidemiology, and End Results (SEER) program.

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As such, continued surveillance of these men is warranted, said lead investigator Elizabeth J. Davis, MD, in a presentation at the American Society of Clinical Oncology annual meeting in Chicago. Study findings were recently published in Cancer (2014; 120:2735-41).

“Surveillance even 10 years after treatment is still very important because of the increased risk of bladder and rectal cancer,” said Dr. Davis, a fellow in the division of hematology/oncology, department of internal medicine at the University of Michigan Health System, Ann Arbor, working with Kathleen A. Cooney, MD, and colleagues.

Previous studies in which second malignancies after treatment for prostate cancer were examined also found an increased risk for cancer of the bladder and rectum but had lag times of only 2 to 5 years, Dr. Davis explained.

“Biologically, we thought that 10 years out is a reasonable time to look at the effects from radiation,” she said.

The relative survival of men diagnosed with localized prostate cancer is 98% at 10 years and 91% at 15 years. The long life expectancy exposes them to potential second primary malignancy.

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EBRT linked with rectal, bladder cancers

Using the SEER registry, Dr. Davis and co-authors identified 96,811 men diagnosed with a first primary, microscopically confirmed malignant prostate cancer between 1992 and 2010 and who had at least 10 years of follow-up. The relative risk of a second primary malignancy was calculated based on the incidence in the general population compared to men who did or did not receive EBRT as their initial treatment for prostate cancer. New primary cancers diagnosed 10 years or more after the original date of the prostate cancer diagnosis were considered.

One-third of the eligible patients had some form of radiation as primary treatment for their prostate cancer; 71.8% had EBRT and 28.2% had radioactive seed implants or unspecified other therapies. Only patients who received EBRT were included in the analysis.

Using standardized incidence ratios for second primaries, men who were treated with EBRT were 70% more likely to be diagnosed with rectal cancer and 42% more likely to be diagnosed with bladder cancer compared with the general population. Men treated with EBRT were significantly more likely than men not treated with radiation to be diagnosed with rectal cancer (p<.0001) and bladder cancer (p<.0001).

Both groups were at reduced risk of second primary malignancy and second solid tumors overall, compared with the general population, but the magnitude of the reduction was less among men who received radiation compared with men who did not receive radiation.

“We’re not sure why men with prostate cancer would be at a decreased risk of a second cancer overall, but we see that both in the patients who received radiation and those who did not. It’s an unexplained finding, but it’s real, with small confidence intervals in a fairly large population,” said Dr. Davis.

Men who have received EBRT for treatment of prostate cancer should be promptly evaluated for symptoms that could represent a second cancer of the bladder and rectum, she concluded.

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