Association between breast cancer and thyroid cancer: A study based on 13 978 patients with breast cancer

BACKGROUND: Thyroid cancer (TC) is one of the most commonly seen secondary malignancy in breast cancer (BC) survivors. MATERIALS AND METHODS: A retrospective study was conducted in BC patients in our center from 1999 to 2013. Patients were divided into BC‐TC group and BC‐alone group. RESULTS: In total, 13 978 BC patients were identified, among whom 247 (1.8%) had TC. The standardized incidence ratio (SIR) of TC was 4.48 compared with Chinese females, and up to 98.0% of cases were thyroid papillary carcinomas. A family history of malignancy was the only independent risk factor (odds ratio = 1.457, P = 0.025) for development of TC in patients with BC. We also identified inferior survival in patients with synchronous versus metachronous BC‐TC (P = 0.016). Synchronous BC‐TC (risk ratio = 5.597, P = 0.018) was an independent prognostic factor for inferior RFS. CONCLUSIONS: We observed high co‐occurrence of TC in patients with BC. There might be different mechanisms behind synchronous and metachronous BC‐TC.