Age at Endometriosis Diagnosis Is Linked to Breast Cancer Risk

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NEW YORK (Reuters Health) Apr 11 - Women who are diagnosed with endometriosis at a young age tend to have a reduced risk for breast cancer, according to researchers, while women who are older when they receive a diagnosis of endometriosis have an increased breast cancer risk.

"Although several risk factors are common to endometriosis and breast cancer, the results of observational studies of an association have so far been inconsistent," Dr. Lisbeth Bertelsen, of the Institute of Cancer Epidemiology, Copenhagen, Denmark, and colleagues write in the March 15th issue of the International Journal of Cancer.

The researchers used data from a large population-based case-cohort study that included data on breast, ovary, and endometrial cancers from the Danish nationwide cancer and hospital registries. A total of 114,327 women were included in the study. Of these, 1978 women had been diagnosed with endometriosis and 16,983 women had been diagnosed with breast cancer between 1978 and 1998.

The women were followed for an average of 17.8 years. Of the women who received a diagnosis of endometriosis, 236 were subsequently diagnosed with breast cancer. The crude rate ratio for breast cancer after endometriosis was 1.00. The rate ratio remained relatively unchanged (RR, 0.97) after adjustment for parental status, age at birth of first child, parity, benign breast disease, bilateral oophorectomy, and calendar year.

The breast cancer risk increased with age at diagnosis of endometriosis. Women who were diagnosed with endometriosis at a young age (younger than 40 years of age) had a reduced breast cancer risk, while those diagnosed with endometriosis at older ages (older than 40 years of age) tended to have an increased breast cancer risk.

"Common risk factors for postmenopausal endometriosis and breast cancer or a stronger effect of altered endogenous estrogen associated with endometriosis diagnosed in older women, who have a naturally low level of endogenous estrogen, might explain the excess of breast cancer in women diagnosed with endometriosis later in life," Dr. Bertelsen's team concludes. "The reduced risk for breast cancer of young women might be related to the antiestrogenic effect of the drugs (Danazol, GnRH agonists), which are used to treat primarily younger women with endometriosis."

Int J Cancer 2007;120:1372-1375.