There is nearly a 40% increased risk for endometrial or thyroid cancer among women with endometriosis, according to a systematic literature review and meta-analysis in Clinical Reviews in Oncology/Hematology.
The 32 studies which were included also found an inverse association with cervical cancer.
But there was no link between endometriosisand breast cancer or melanoma.
The studies were conducted in 13 different countries and published between 1989 and 2018, of which 17 had a cohort design, 13 were case-control studies (10 population-based and 3 hospital-based) and 2 had a cross-sectional design.
The diagnosis of endometriosis was deemed confirmed in 16 of the studies.
The methodological quality of the studies was rated “fair” or “good” for all cohort studies, for 9 of the case-control studies and for 1 of the cross-sectional studies.
The most likely reasons to judge a study’s methodological quality as poor were the inability to adjust for age and other confounding factors or reliance on self-reported endometriosis diagnosis.
“There was no evidence that women with endometriosis were at increased risk of developing breast cancer,” the Italian authors wrote, “despite the frequent hormonal imbalance with excess oestrogen production (a known risk factor for breast cancer) in these patients.”
The summary relative risk (SRR) was 1.04 (95% CI: 0.99 to 1.09), with between-estimates heterogeneity slightly above the range of acceptable values.
These breast cancer results were based on 18 independent studies, comprising over 6,000 breast cancer cases of women with endometriosis, of which nearly 4,500 were from three countrywide prospective investigations from Scandinavian countries (one study each from Sweden, Denmark and Finland).
A total of 16 independent studies reported a relative risk estimate for the connection between endometriosis and endometrial cancer risk, consisting of more than 440 endometrial cancer cases among women with endometriosis, with the finding of an increased risk of the cancer (SRR 1.38; 95% CI:1.10 to 1.74)
Conversely, there was a statistically significant inverse association between endometriosis and cervical cancer risk (SRR 0.78; 95% CI: 0.60 to 0.95), based on estimates from four prospective studies and one cross-sectional study.
A relative risk estimate for the link between endometriosis and thyroid cancer risk was reported in five independent studies, all of which had a cohort design and concluded a generally moderate positive association (SRR 1.38; 95% CI: 1.17 to 1.63).
The authors noted that unbalanced oestrogen metabolism and autoimmunity might explain this 38% excess risk.
In addition, reproductive factors may play a major role in thyroid carcinogenesis, as the peak incidence of papillary thyroid cancer occurs between the ages of 40 to 49 in women.
For the association between endometriosis and melanoma risk, the SRR was 1.30 (95% CI: 0.86 to 1.96), based on 10 independent studies consisting of more than 500 melanoma cases of women with endometriosis.
One study also reported a statistically significant association between endometriosis and the risk of basal cell cancer (RR 1.18; 95% CI: 1.10 to 1.25).
Concerning other malignancies, three cohort studies reported increased risk estimates for non-Hodgkin lymphoma (NHL).
The authors said their overall findings need to be confirmed by appropriately designed cohort studies with clinical confirmation of endometriosis.
Furthermore, “the role of the immune system in the pathogenesis of endometriosis and its co-morbidities (including cancer) and the biological mechanisms underlying these associations remain to be elucidated,” the wrote.