Petah Tikva, Israel—Radiation therapy for prostate cancer may increase the risk of a potentially aggressive form of rectal cancer, Israeli investigators reported.

Compared with the general population, prostate cancer patients treated with radiation therapy were almost twice as likely to develop rectal cancer.

Further, compared with radical prostatectomy, radiation therapy also increased the risk of rectal cancer, David Margel, MD, a urologist at Rabin Medical Center in Petah Tikva, said in a presentation at the AUA annual meeting in Chicago. A substantial proportion of the patients who developed rectal cancer after radiation therapy had symptoms at diagnosis, and one-fourth of the patients subsequently died of rectal cancer.

Radiation therapy increases the risk of secondary malignancies in the irradiated field, most notably bladder cancer. Currently, no consensus exists as to whether the risk extends to the rectum.

To explore associations between pelvic radiation and rectal cancer, Dr. Margel and colleagues queried the Israel Cancer Registry to identify all cases of prostate cancer diagnosed between 1980 and 2005. They identified 29,593 patients, including 2,163 treated with radiation therapy and 6,762 treated surgically.

The analysis showed that 194 patients (0.7%) developed rectal cancer a year or more after diagnosis of prostate cancer. During follow-up of 11,597 person-years, 26 patients treated with radiation therapy (1.2%) developed rectal cancer. In contrast, 41 patients treated with radical prostatectomy (0.6%) developed rectal cancer during follow-up of 27,096 person-years.

Total rectal cancer incidence in Israel for the study period was 12 per 100,000 patient-years. Overall, prostate cancer patients had a risk of rectal cancer similar to that of the general population, reflected in a standardized incidence ratio (SIR) of 0.9.

The subset of prostate cancer patients treated with radiation therapy had an SIR of 1.81 compared with that of the general population (95% confidence interval [CI]: 1.2–2.5). Radiation therapy also increased the risk of rectal cancer when compared to the risk for men whose cancer was treated surgically (SIR=1.22, 95% CI: 1.65–0.85).

Characteristics of the patients who developed rectal cancer after pelvic irradiation included a median age of 69 at prostate cancer diagnosis, low or intermediate risk in 18 of 26 cases, median time from treatment of prostate cancer to diagnosis of rectal cancer of 6 years, median rectal cancer location of 7 cm from anal verge, and median follow-up of 10 years from diagnosis of rectal cancer.

Four of the 26 cancers (0.2%) were diagnosed during screening colonoscopy, and four were diagnosed during elective colonoscopy in response to patient complaints of constipation. Thirteen patients were diagnosed following hospital admission for obstruction or rectal bleeding.

At diagnosis of rectal cancer, 39% of the patients had locally advanced disease, 15% had lymph node involvement, and 15% had metastatic cancer. During a median follow-up of 10 years, 27% of the patients died of rectal cancer, 34% died of other causes, 4% died of prostate cancer, and 35% were alive.