Younger women with endometriosis seem to be at a higher risk for breast cancer, but several factors could be contributing to this finding.
The overall risk of breast cancer in women with surgically verified endometriosis is comparable to that in the general population, according to a recent Finnish population-based cohort study.
But the Finnish study also found that risk of breast cancer is increased in women under age 40 who have endometriosis.
In addition, younger women with surgically verified endometriosis are highly symptomatic and have more frequent surgeries and multiple therapies, which contribute to their risk of breast cancer.
“These results are not really a surprise. Endometriosis and breast cancer share similar risk factors like early menarche and nulliparity, which might also explain our findings in part,” said principal investigator Oskari Heikinheimo, MD, PhD, a professor of ob/gyn at the University of Helsinki, who has had a long-time interest in endometriosis.
The study in the journal Acta Obstetrica et Gynecologica Scandinavica captured records of 49,933 women with a first endometriosis-associated diagnosis occurring concomitant with relevant surgical codes from the Finnish Hospital Discharge Register between 1987 and 2012.
Among the cohort, there were 23,210 cases of ovarian endometriosis, 20,187 cases of peritoneal endometriosis, 2,372 cases of deep infiltrating endometriosis, 1,120 cases of mixed endometriosis and 3,044 cases of other or unknown, according to the site of the endometriosis.
The Finnish female population served as the reference population.
There were 1,555 cases of breast cancer in the cohort, which was similar to the reference population (standardized incidence ratio [SIR] 0.99; 95% CI: 0.94 – 1.03). The incidence and type of endometriosis also did not differ between the groups, and were comparable for ductal and lobular breast cancer.
However, women between ages 20 and 29 with surgically verified endometriosis were more than four times as likely to develop breast cancer (SIR 4.44; 95% CI: 2.22 – 7.94), although the number was very small (n = 11).
Cohort women between ages 30 and 39 were also at increased risk (SIR 1.28; 95% CI: 1.03 – 1.57).
Similarly, the 170 cases of in situ breast cancer in the cohort presented increased risk (SIR 1.25; 95% CI: 1.07 – 1.44)
“Because early onset of breast cancer is rare, the absolute excess risk from our study is small,” Dr. Heikinheimotold Contemporary OB/GYN. “For instance, in women between the ages of 20 and 29, the relative risk of 4.4 translates into only eight additional breast cancer cases among 10,000 women with endometriosis who are followed for 5 years.”
Likewise, in the older age group of 30 to 39 years, the corresponding number is an increase of five cases.
“Despite these encouraging numbers, the known risk factors of early onset breast cancer account for only about 10% of such cases,” Dr. Heikinheimosaid. “We speculate that genetic factors predisposed to breast cancer might be more prevalent among women with endometriosis at early age.”
Low body mass index and oral contraceptive use are two other potential risk factors for breast cancer in young women.
Dr. Heikinheimo and his colleagues have analyzed the risks of gynecological and non-gynecological cancers in the same cohort in a large study that was published last year in Obstetrics & Gynecology and theInternational Journal of Cancer. The study concluded that endometriosis was linked to a nearly two-fold risk of ovarian cancer, especially with endometrioid and clear cell histologic type.
“Going forward, we will consider in more detail the factors affecting the cancer risks in women with surgically diagnosed endometriosis, including nulliparity and the types of treatments administered,” Dr. Heikinheimo said.
Dr. Heikinheimo reports no relevant financial disclosures.