Endometrial cancer following treatment for breast cancer: a case-control study in Denmark.

To evaluate the risk of endometrial cancer subsequent to breast cancer, a case-control study was carried out in Denmark. Between 1943-1977, 115 cases of histologically confirmed endometrial carcinoma developed more than 3 months after the diagnosis of a primary breast cancer in 51,638 women. A total of 235 breast cancer patients with no second primary cancer were matched to the cases on age, calendar year of diagnosis, and survival with an intact uterus. Identification of cases and controls relied upon records available in the Danish Cancer Registry. Information on risk factors and reproductive histories was abstracted from hospital records. Increased relative risks (RR) for endometrial cancer were associated with menopausal oestrogen use (RR = 4.9), nulliparity (RR = 2.1), late age at natural menopause (RR = 2.9), and pelvic irradiation (RR = 1.4). No association was apparent for drugs used to treat breast cancer. This study indicates that breast and endometrial cancer share several common aetiologic factors and that studies of second primary cancers have the potential to provide information on risk factors other than those associated with therapy.