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. In the current study, we investigated the association of thyroid cancer incidence in a cohort of 13 978 breast cancer patients. We identified that the standardized incidence ratio of secondary thyroid cancer was significantly increased and a family history of malignancy was the only independent risk factor, which provided a new insight into the association of breast cancer and thyroid cancer. [ABSTRACT FROM AUTHOR]