Risks of gynecological and breast cancers in women with endometriosis or adenomyosis: A nationwide cohort study

INTRODUCTION: Endometriosis has been associated with several risk factors for cancer, and population-based research has shown that women with endometriosis have a higher risk of several types of cancer. The aim of this study was to examine the risks of gynecological and breast cancers among women diagnosed with endometriosis or adenomyosis. MATERIAL AND METHODS: A nationwide cohort study was conducted between 1997 and 2018. All women diagnosed with endometriosis or adenomyosis (the first recorded diagnosis during the study period) in Sweden were followed for cancer diagnosis. The reference population was women not diagnosed with endometriosis or adenomyosis during the study period. Data were obtained from the National Patient Register (in patient and out patient specialist care settings), the Swedish Cancer Register, and the Total Population Register. Adjusted standardized incidence ratios with 95% confidence intervals were calculated for cancer. Women with prior gynecological organ removal or mastectomy were not excluded. RESULTS: A total of 49 133 women were diagnosed with endometriosis or adenomyosis at a mean age of 42.7 years (standard deviation: 9.97 years). Of these, 1784 (3.6%) were diagnosed with gynecological or breast cancer. The overall standardized incidence ratio for gynecological and breast cancer in women with endometriosis or adenomyosis for the whole study period was 1.4 (95% confidence interval, 1.3-1.5) compared to those without. For cancers diagnosed in the same calendar year as endometriosis or adenomyosis (n = 370), the standardized incidence ratios were particularly high for endometrial cancer (standardized incidence ratio 47.0) and ovarian (standardized incidence ratio 39.6) cancer. For cancers diagnosed in subsequent calendar years (n = 1414), the standardized incidence ratios were 0.6 (0.5-0.8) for endometrial cancer, 0.7 (0.5-0.9) for cervical cancer, 1.2 (1.1-1.3) for breast cancer, and 1.9 (1.6-2.2) for epithelial ovarian cancers overall. When looking at ovarian cancer subtypes, the standardized incidence ratios were 1.5 (1.1-2.0), 2.8 (1.7-4.2), and 7.7 (5.4-10.6), for low-grade serous, endometrioid, and ovarian clear cell cancers, respectively. CONCLUSIONS: Women with endometriosis or adenomyosis have a higher subsequent risk of ovarian and breast cancers compared to women without. This may represent important information for clinicians and a foundation for further studies on prophylactic interventions.