The Risk of Cancer and the Role of Parity Among Women With Endometriosis

Epidemiologic studies have suggested that women with endometriosis are at increased risk of cancer—particularly ovarian cancer and non-Hodgkin lymphoma. These women also are at increased risk of being infertile, and both infertility and nulliparity are acknowledged to be risk factors for ovarian and other cancers. This study examined the risk of cancer in women with endometriosis and the influence of parity on cancer risk. Participants were 63,630 women listed in the National Swedish Inpatient Register who were discharged from hospital with a diagnosis of endometriosis in the years 1969 to 2002. Excluding the first year of follow-up in order to rule out cancers present at the time endometriosis was diagnosed left 3822 cases of cancer. Follow-up averaged 13.4 years. No significantly increased overall risk of cancer was found; the standardized incidence ratio (SIR) was 1.01. There were, however, increased risk estimates for endocrine tumors (SIR, 1.38), ovarian cancer (SIR, 1.37), renal cancer (SIR, 1.36), thyroid cancer (SIR, 1.33), brain tumors (SIR, 1.27), malignant melanoma (SIR, 1.23), and breast cancer (SIR, 1.08). The risk of cervical cancer was reduced in women with endometriosis (SIR, 0.71). In no instance was there a significant difference in cancer risk between parous and nulliparous women. The risk of ovarian cancer declined with increasing parity, but not to a statistically significant degree. The risk of endometrial cancer was increased in women who had given birth to two children only (SIR, 1.60). The risk of malignant melanoma was highest in nulliparous women (SIR, 1.47). This study confirmed that women with endometriosis are at increased risk of developing several types of cancer and that the increased risk of ovarian cancer is not a function of parity.