Introduction/Background Breast cancer (BC) is a recognised risk factor for endometrial cancer (EC). Emerging literature indicates it confers a higher risk of type II EC (T2EC) than type I EC (T1EC). Some surgeons offer a prophylactic hysterectomy to BC patients referred for risk-reducing bilateral salpingo-oophorectomy yet insufficient evidence prevents this from being standard practice. We aimed to quantify their relative and absolute risks of developing both endometrial cancer subtypes and identify a higher-risk group that should be considered for prophylactic hysterectomy.

Methodology This retrospective observational case-control study compared patients diagnosed with BC between 2008 and 2014, who subsequently developed EC within 10 years to those who did not. Absolute and relative risks were calculated using the numbers of regional BC and EC cases within this group, alongside 2009 UK female population and EC incidence statistics. Binary logistic regression generated adjusted odds ratios for patient- and disease-specific variables.

Results Forty-five BC patients developed endometrial cancer; twenty-four had T1EC and twenty-one had T2EC. Their relative risk of developing EC was greater than the general population’s (RR 10.24, p<0.0001). Notably, this was higher for T2EC (RR 32.37, p<0.001) than T1EC (RR 6.12, p<0.0001). Nonetheless, the absolute risk remained low. Tamoxifen exposure was significantly more prevalent amongst T2EC patients (adjusted odds ratio 79.61, p=0.003). Increased age at BC diagnosis was associated with T1EC (adjusted odds ratio 1.10, p=0.043) and T2EC (adjusted odds ratio 1.13, p=0.03). Neither smoking status nor family history of BC was significantly associated with any outcome.

Conclusion Women with BC were more likely to develop T2EC than T1EC. The absolute risk was low, but the cumulative risk was substantial enough to consider prophylactic hysterectomy in the premenopausal group. Tamoxifen exposure was significantly predictive of EC, particularly T2EC, and might facilitate risk estimation. Older women at BC diagnosis who received tamoxifen treatment should be screened and closely monitored for EC. Clarification of the menopausal status will help make more meaningful recommendations.

Disclosures All authors state that they do not have any conflicts of interest.