Endometriosis and the risk of cancer with special emphasis on ovarian cancer

BACKGROUND: Several observations of the coexistence of endometriosis and cancer have been published. One study concerning endometriosis patients from 1969 to 1986 showed an overall relative cancer risk of 1.2 and relative risks for breast cancer, ovarian cancer and non-Hodgkin's lymphoma to be 1.3, 1.9 and 1.8, respectively. The aim of this study was to see whether these risk ratios stand in an extended study with longer follow-up. METHODS: Women discharged from a hospital, with a diagnosis of endometriosis from 1969 to 2000, were identified using the National Swedish Inpatient Register. Data were linked to the National Swedish Cancer Register to identify cases of cancer. Data on hysterectomies and oophorectomies were available. Standardized incidence ratios (SIR) were calculated. RESULTS: 64 492 women entered the study. First year of follow-up was excluded, leaving 3349 cases of cancer. There was no increased overall risk of cancer [SIR 1.04, 95% CI 1.00-1.07]. Elevated risks were found for ovarian cancer (SIR 1.43, 95% CI 1.19-1.71), endocrine tumours (SIR 1.36, 95% CI 1.15-1.61), non-Hodgkin's lymphoma (SIR 1.24, 95% CI 1.02-1.49) and brain tumours (SIR 1.22, 95% CI 1.04-1.41). Women with early diagnosed and long-standing endometriosis had a higher risk of ovarian cancer, with SIR of 2.01 and 2.23, respectively. The average age at endometriosis diagnosis was 39.4, indicating that there are the moderate/severe cases that are included in this study. Women who had a hysterectomy before or at the time of the endometriosis diagnosis did not show an increased risk of ovarian cancer. CONCLUSION: Women with endometriosis have an increased risk of some malignancies, particularly ovarian cancer, and the risk increases with early diagnosed or long-standing disease. Hysterectomy may have a preventive effect against ovarian cancer.