Risk of Second Primary Malignancy in Breast Cancer Survivors: A Nested Population-Based Case-Control Study

PURPOSE: Evolving therapies have improved the prognoses of patients with breast cancer; and currently, the number of long-term survivors is continuously increasing. However, these patients are at increased risk of developing a second cancer. Thus, late side effects are becoming an important issue. In this study, we aimed to investigate whether patient and tumor characteristics, and treatment type correlate with secondary tumor risk. METHODS: This case-control study included 305 patients with a diagnosed second malignancy after almost 6 months after the diagnosis of primary breast cancer and 1,525 controls (ratio 1:5 of cases to controls) from a population-based cohort of 6,325 women. The control patients were randomly selected from the cohort and matched to the cases according to age at diagnosis, calendar period of diagnosis, disease stage, and time of follow-up. RESULTS: BRCA1 or BRCA2 mutation, human epidermal growth factor receptor 2 (HER2)+ status, chemotherapy, and radiotherapy were related to increased risk of developing a second cancer, whereas hormonotherapy showed a protective effect. Chemotherapy, radiotherapy, and estrogenic receptor level <10% increased the risk of controlateral breast cancer. HER2+ status increased the risk of digestive system and thyroid tumors, while BRCA1 or BRCA2 mutation increased the risk of cancer in the genital system. CONCLUSION: Breast cancer survivors are exposed to an excess of risk of developing a second primary cancer. The development of excess of malignancies may be related either to patient and tumor characteristics, such as BRCA1 or BRCA2 mutation and HER2+ status, or to treatments factors.