Studies have shown an increased risk of malignancies in women with endometriosis. Little is known about the impact of endometriosis on cancer survival. We investigated whether the survival after a diagnosis of a malignancy differs in women with a previously diagnosed endometriosis compared to other women. Women with a first time diagnosis of a malignancy in 1969-2005, were identified using the National Swedish Cancer Register (NSCR). By use of the National Swedish Patient Register (NSPR) we identified all women with a diagnosis of endometriosis during the same period and linked these patients with the data from the NSCR. The cohort comprised 4,278 women with endometriosis and a malignancy, and 41,831 randomly selected matched women without endometriosis. Cox regression was used for all calculations to obtain crude and adjusted cause specific mortality rates, measured as hazard ratios (HR) with 95% confidence intervals (CI). A total of 46,109 women entered the study. There was a statistically significant better survival for women with endometriosis for all malignancies combined (HR=0.92) and for breast cancer (HR=0.86) and ovarian cancer (HR=0.81) specifically. For breast cancer the survival enhancing effect in women with endometriosis decreased with increasing parity. There was poorer survival in malignant melanoma for women with endometriosis (HR=1.52). The survival in a malignancy is better in women with a previously diagnosed endometriosis compared to women without endometriosis especially for breast and ovarian cancers. The prognosis of malignant melanoma is poorer in women with endometriosis.
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