Patients with a previous cancer history (PCA) are routinely excluded from most clinical trials, which may limit the accuracy and universality of clinical trials. We aimed to explore the association between PCA and survival of patients with different molecular subtypes of breast cancer. Patients diagnosed with breast cancer from the Surveillance, Epidemiology, and End Results (SEER) database between 2010 and 2015 were included in this retrospective cohort study. The primary outcome was overall survival (OS), which was calculated from date of diagnosis to date of death or censor date during this period. The relationship between PCA and OS of patients with different molecular subtypes of breast cancer was analyzed by the Kaplan-Meier curves and multivariate Cox proportional-hazards model. A total of 35,640 primary breast cancer patients were included, and 2,038 (5.72%) patients had a PCA. Female genital system cancer (491 cases, 24.09%) was the largest proportion type of previous cancer, and HER2-positive (24,754 cases, 69.46%) breast cancer was the most common subtype. Patients with previous female genital/endocrine system cancer history and other cancers history were associated with a poorer OS in overall patients, and in patients with triple-negative and HER2-positive subtypes (P < 0.05). In patients with Luminal A and Luminal B subtypes, previous other cancers history was related to poor OS (P < 0.05), while female genital/endocrine system cancer history may not influence the OS (P > 0.05). Subgroup analyses presented that PCA was related to poor OS in patients aged 40-64 years and ≥65 years (P <0.05), while prognosis in patients aged 18-40 years may not be influenced by PCA (P > 0.05). The impact of PCA on the prognosis of breast cancer patients was related to molecular type, patient age, and type of PCA. In clinical trials of breast cancer, the exclusion criteria for PCA patients may be modified according to the above variables.
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