Increased second primary liver cancer risk in young breast cancer patients undergoing radiotherapy and chemotherapy: a nationwide population-based study

Background: Although chemotherapy (CT) and radiotherapy (RT) are important breast cancer (BC) treatment modalities, they can cause other cancers. However, second cancers of liver and stomach tend to be ignored during BC treatment. In this study, the incidence patterns of second primary cancer of liver and stomach were analyzed. Materials and Methods: The insurance claim data of patients that underwent definitive surgery from 2009 to 2010 were analyzed. Standardized incidence ratios (SIRs) were used to estimate the relative risks of second primary liver cancer (SPLC) and second primary stomach cancer (SPSC). In addition, hazard ratios (HRs) of risk factors were analyzed. Data were obtained on 21,024 BC patients that underwent definitive mastectomy. Results: The median follow-up period was 105.5 months. Overall SIRs for SPLC and SPSC were 7.26 (p<0.01) and 2.92 (p<0.01). In the young age group (aged 20–39 years), the crude HR for CT was 2.27 (p < 0.05) and the age/RT adjusted HR was 2.37 (p<0.05). RT also showed a tendency to induce SPLC. The effect of CT peaked within 5 years after treatment, whereas the effect of RT gradually increased after 5 years. Conclusion: This study shows CT and RT both increase the risk of SPLC in BC patients and that these increases are greater in young BC patients. Times to SPLC occurrence after RT and CT were found to differ.