SHORT REPORT Risk for breast cancer among women with endometriosis

Although several risk factors are common to endometriosis and breast cancer, the results of observational studies of an association have so far been inconsistent. We evaluated the relationship between endometriosis and breast cancer on the basis of data on selected cancers and medical histories from the Danish nationwide cancer and hospital registries used in a large case–cohort study. A total of 114,327 women were included in the study of whom 1,978 women had received a diagnosis of endometriosis and 16,983 had had a diagnosis of breast cancer between 1978 and 1998. Of the women with endometriosis, 236 subsequently received a diagnosis of breast cancer. The crude overall rate ratio for breast cancer after endometriosis was 1.00 and after adjustment for reproductive factors, calendar-period, bilateral oophorectomy and benign breast disease, the rate ratio was 0.97 (95% confidence interval, 0.85–1.11). The risk for breast cancer increased with age at diagnosis of endometriosis, so that women in whom endometriosis was diagnosed at a young age (approximately <40 years) had a reduced risk for breast cancer and women in whom endometriosis was diagnosed at older ages (approximately 40 years) tended to have an increased risk for breast cancer. The reduced risks observed among young women may reflect their exposure to drugs with antiestrogenic effects. The increased risk associated with endometriosis among postmenopausal women may be due to common risk factors between postmenopausal endometriosis and breast cancer or an altered endogenous estrogen. ' 2006 Wiley-Liss, Inc. A woman’s lifetime exposure to endogenous estrogens is an established risk factor for breast cancer. 1 It is conceivable that a woman’s cumulative exposure to endogenous estrogens is influenced by medical conditions related to hormonal abnormalities that she might have had during her reproductive life. Endometriosis is an estrogen-related, gynecological disorder with an estimated prevalence of 2.5–3.5% in women of reproductive age. 2,3 The condition is characterized by the presence of endometrial tissue in ectopic foci outside the uterus, which can result in chronic pelvic pain, delayed pregnancy and infertility. The cause of endometriosis is unclear, but maintenance of the disease is dependent on the presence of estrogen, 3 as reflected in the characteristics and risk factors related to the condition. Endometriosis is more common among nulliparous than parous women and is rarely seen in women with anovulatory cycles. It presents after menarche and usually regresses after the menopause. Postmenopausal cases of endometriosis are primarily seen in connection with obesity or use of hormone replacement therapy (HRT). 4 Treatment with e.g., danazol or gonadotropin-releasing hormone (GnRH) agonists, which suppress endogenous estrogen production, has proven effective in relieving the symptoms associated with endometriosis. The latter treatment is prescribed mainly to premenopausal women. Although endometriosis and breast cancer appear to have common risk factors such as endogenous estrogen, reproductive characteristics, obesity and use of HRT, which indirectly support an association between the 2 diseases, the results of observational studies of the association between endometriosis and breast cancer are inconsistent. 5–10 Two cohort studies based on hospital records of patients with endometriosis showed an increased risk for breast cancer, 6,7 while 4 other studies based on selfreports of endometriosis did not confirm such an association. 5,8–10 To clarify further the relationship between breast cancer and endometriosis, we used data from a large population-based case–cohort study with data from Danish registers and designed to examine associations between various medical conditions and breast, ovary and endometrial cancer.