Risk Factors of Developing Contralateral Breast Cancer After Treatment of a First Primary Breast Cancer

Introduction Breast cancer (BC) is the most common cancer among women worldwide. This study aimed to determine the disease-free interval (DFI) and the effect of risk factors and characteristics of first primary breast cancer survivors on the progression of contralateral breast cancer (CBC). Method In this retrospective cohort study, we identified 5003 women (containing 145 CBC and 4858 BC survivors) diagnosed with BC between 2000 and 2020 in the cancer research center. CBC was diagnosed at least 6 months after detection of primary BC. Kaplan–Meier method was used to indicate DFI and survival curves. To determine the risk factors of CBC, the Cox proportional hazard regression model was employed. Results The median time interval among primary BC and CBC was 7.10 years (average: 7.57, range: 0.49-19.95). The 5-year DFI was 97%. The median age of CBC and primary BC patients was 47 (range 17-78) and 48 (range 17-90), respectively. ER+, PR+, and HER2+ were reported in 72.13%, 66.67%, and 30% of CBC patients. Also, 69.57% of patients had IDC pathology type and 81.90% and 83.64% of the patients were treated with adjuvant chemotherapy and external radiotherapy. More than half of the participants had no family history of BC (69.57%). The hazard ratio indicates that women 60-70 years old, a higher number of involved lymph nodes, and recurrence had significant relationships with CBC. Conclusion This is the first study to investigate the risk factors of CBC and disease-free interval among BC survivors. Women with higher lymph node metastasis have a lower chance to develop CBC and patients with recurrence are at higher risk for CBC.