[Prognostic analysis of double primary breast cancer and endometrial cancer patients based on SEER database]

Objective: To investigate the survival outcomes and prognostic factors of patients with double primary breast cancer (BC) and endometrial cancer (EC). Methods: A retrospective cohort study was conducted using data for the period 1992-2018 from the Surveillance, Epidemiology, and End Results (SEER) database. There were 3 465 patients with BC as the first primary cancer (BC-EC group) and 2 804 patients with EC as the first primary cancer (EC-BC group). Kaplan-Meier analysis and cumulative incidence function were used to estimate overall mortality, breast cancer-specific mortality, and endometrial cancer-specific mortality, respectively. Cox regression and Fine-Gray regression were used to analyze the prognostic factors of overall mortality, breast cancer-specific mortality, and endometrial cancer-specific mortality, respectively. Results: During a median follow-up of 160 months, 1 616 deaths occurred in the BC-EC group, with EC being the leading cause of death (37.69%); 994 deaths occurred in the EC-BC group, with BC being the leading cause of death (28.77%). Cox regression identified patients with older ages at first primary cancer diagnosis (54-61 years: HR=1.46, 95% CI: 1.26-1.69; 62-68 years: HR=2.64, 95% CI: 2.29-3.03; ≥69 years: HR=4.89, 95% CI: 4.27-5.60), shorter time interval between the diagnoses (0-5 months: HR=6.13, 95% CI: 5.21-7.21; 6-23 months: HR=5.69, 95% CI: 4.95-6.55; 24-59 months: HR=3.44, 95% CI: 3.04-3.89; 60-119 months: HR=2.32, 95% CI: 2.07-2.59), mixed ductal-lobular BC (HR=1.29, 95% CI: 1.11-1.48), endometrial mixed cell adenocarcinoma (HR=1.23, 95% CI: 1.01-1.50), advanced tumor grade (grade Ⅱ BC: HR=1.13, 95% CI: 1.01-1.27; grade Ⅲ BC: HR=1.24, 95% CI: 1.10-1.41; grade Ⅱ EC: HR=1.19, 95% CI: 1.06-1.33; grade Ⅲ EC: HR=1.68, 95% CI: 1.48-1.90), advanced tumor stage of the two cancers (distant BC: HR=3.14, 95% CI: 2.50-3.94; regional EC: HR=1.53, 95% CI: 1.36-1.71; distant EC: HR=3.00, 95% CI: 2.59-3.47) had increased risk of overall mortality. Fine-Gray regression showed that compared with BC-EC patients, EC-BC patients had a higher risk of breast cancer-specific mortality [sub-distribution hazard ratio (sHR=1.24, 95% CI: 1.04-1.47], but a lower risk of endometrial cancer-specific mortality (sHR=0.37, 95% CI: 0.30-0.46). Older ages at first cancer diagnosis, shorter intervals between the diagnoses, negative ER and PR status, and advanced BC grades/stages were associated with increased breast cancer-specific mortality (P＜0.05). Similarly, older ages, shorter intervals, endometrial serous carcinoma/mixed cell adenocarcinoma, and advanced EC grades/stages correlated with elevated endometrial cancer-specific mortality (P＜0.05). Conclusion: The management of double primary BC and EC patients requires multidisciplinary strategies, with particular attention to patients presenting older ages at first cancer diagnosis, shorter intervals between the diagnoses, and unfavorable tumor characteristics.