Incidence of second primary malignancies in women with different stages of breast cancer

Introduction: Breast cancer (BC) is the most common cancer in women worldwide. Because of the extended survival of patients with BC, the occurrence of second primary malignancies (SPMs) after BC is an important issue. Methods: We identified female patients with BC in the Breast Cancer Health Database of Taiwan, which includes four cancer registry datasets between 2002 and 2014 from Taiwan Cancer Registry. We compared the incidence of SPM between patients who received chemotherapy and/or radiotherapy with those who did not. Stratified analyses were performed according to the American Joint Committee on Cancer (AJCC) stage. The Cox regression model was used to identify the risk factors for SPM and evaluate their effects. Results: We enrolled 85,947 eligible patients with BC, and 2,656 (3.09%) patients developed SPM. The median duration of SPM was 2.70 (1.14-5.14) years. Radiotherapy was administered in 40,946 (47.64%) patients, and chemotherapy was administered in 52,120 (60.64%). The most common SPMs were digestive tract cancers (876, 31.89%). The risk factors for SPM included the AJCC stage, chemotherapy, radiotherapy, age, and underlying comorbidities. Neither chemotherapy nor radiotherapy was associated with an increased risk of SPM in any stage. In contrast, after adjusting for other risk factors, patients at stage III/IV who received both therapies had lower risks of SPM compared with those who did not (p = 0.047). Conclusion: The risk of SPM was different across BC stages. Neither chemotherapy nor radiotherapy was associated with an increased risk of SPM in women with BC.