Analysis and prediction of second primary malignancy in patients with breast cancer

Second primary malignancy (SPM) is common in breast cancer (BC). The present study aimed to profile the characteristics of BC with SPM and to identify patients at high risk of SPM. Clinical and outcome data of BC cases were retrieved from the SEER database. Principal component analysis and a random forest model were utilized to create a model for predicting the occurrence of SPMs. Of the 286,047 BC cases analyzed, 9.32% developed SPMs. Approximately 70% of BCs that developed SPMs were ductal carcinoma and 71% of BCs that developed SPMs were human epidermal growth factor receptor 2 (HER2)^-/hormone receptor (HR)⁺. The overall survival (OS) of the SPM cohort was significantly worse (hazard ratio: 1.49; 95% CI: 1.44-1.53; log-rank P<0.001). After adjusting for metastasis status, SPM was still a poor prognostic factor (hazard ratio: 1.71; 95% CI: 1.70-1.82; log-rank P<0.001). Of note, 50.5% of the SPMs occurred in the breast and the OS of the breast SPM group was significantly better than that of the other single-organ SPM group (hazard ratio: 0.46; 95% CI: 0.45-0.49; log-rank P<0.001) and the multiple-organ SPM group (hazard ratio: 0.44; 95% CI: 0.39-0.50; log-rank P<0.001). A random forest model created from clinical features predicted SPM with a positive predictive value of 32.3% and negative predictive value of 90.7% in the testing set. Thus, SPM occurs in nearly 1/10 of BC survivors and its existence and occurrence site significantly influence OS. SPM may be partly predicted from clinical features. In addition, it was indicated that postmenopausal elderly patients with a HER2^-/HR⁺ molecular subtype should be more watchful and undergo screenings for SPMs.