The risk of cancer and the role of parity among women with endometriosis.

BACKGROUND Several epidemiological studies have shown an increased cancer risk among women with endometriosis, especially ovarian cancer. Infertility and nulliparity are also known risk factors for different types of cancer. The aim of this study is to investigate cancer risk among women with endometriosis, stratifying for parity. METHODS Women discharged from a hospital, with the diagnosis of endometriosis from 1969 to 2002, were identified using the National Swedish Inpatient Register. Data were linked to the National Swedish Cancer Register to identify cases of cancer and to the Swedish Multi-Generation Register to calculate parity and age at first birth. Standardized incidence ratios (SIR) were calculated. RESULTS A total of 63,630 women entered the study. To exclude cancers already present at the time of endometriosis diagnosis, the first year of follow-up was excluded, leaving a number of 3,822 cases of cancer. There was no increased overall risk of cancer (SIR 1.01) among women with endometriosis. Endometriosis was associated with elevated risks for endocrine tumours (SIR 1.38), ovarian cancer (SIR 1.37), renal cancer (SIR 1.36), thyroid cancer (SIR 1.33), brain tumours (SIR 1.27), malignant melanoma (SIR 1.23) and breast cancer (SIR 1.08), as well as a reduced risk for cervical cancer (SIR 0.71). There were no significant differences between nulliparous and parous women with endometriosis regarding cancer risk for any of the cancer types. There was a non-significant decrease in risk of ovarian cancer with increasing parity for women with endometriosis. CONCLUSIONS Women with endometriosis have an increased risk for several malignancies. The increased risks do not seem to be related to parity.