To explore and describe clinicopathological characteristics and prognosis of patients with double primary breast cancer (BC) and thyroid cancer (TC). Medical records of 98 patients diagnosed with double primary breast and thyroid cancer in National Cancer Center (NCC)/Cancer Hospital between January 1, 2001 and December 31, 2020 were retrospectively collected. All of the patients were followed up until January 1, 2021 to acquire survival data. Univariate survival analysis was conducted by Kaplan-Meier method, and multivariate survival analysis was carried out using the Cox proportional hazard model. All of 98 patients in the group were women. The age at diagnosis of the first tumor ranged from 26-72 years old, and the median age was 47 years old. The BC recurring TC (breast methyl) group included 18 cases, TC recurring BC (methyl breast) group included 60 cases, BC and TC simultaneously occurred group (the two are diagnosed within 3 months) included 20 cases. There were statistically significant differences in breast cancer pathological grading, breast cancer postoperative radiotherapy, and combined with other tumors in breast methyl group, methyl breast group and the simultaneous group (0.05). Univariate analysis showed that BC stage and estrogen receptor (ER) were related to overall survival (<0.05), while the family history of BC, BC stage, and ER were not related with the recurrence and metastasis (<0.05). Multivariate analysis showed that BC family history, ER positive, and the order of tumor diagnosis (TC recurring BC) were independent influencing factors for the recurrence and metastasis (<0.05). ER negative is a poor prognostic factor for the double primary breast and thyroid cancer.

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