Risk of Radiation-Associated Contralateral Breast Cancer in Germline Mutation Carriers: A Meta-Analysis and Systematic Review.

Background/Objectives: Contralateral breast cancer (CBC) is a significant concern among breast cancer survivors, particularly in those with moderator-high penetrance germline mutations such as BRCA1, BRCA2, CHEK2, and ATM. While radiotherapy (RT) is a crucial component of breast cancer (BC) treatment, its potential role in increasing CBC risk remains unclear. This systematic review and meta-analysis aim to evaluate the incidence of radiation-induced CBC in germline mutation carriers. Methods: Following PRISMA guidelines, we conducted a comprehensive search in six electronic databases (PubMed, Scopus, Cochrane Library, ProQuest, EBSCO, and Epistemonikos) for studies published fifteen years prior, up to August 2025. We included cohort and case-control studies assessing the association between RT and CBC incidence in germline mutation carriers. A meta-analysis was performed using a random-effects model to estimate cumulative risk (CR) and rate ratios (RR). Results: Seven studies were included. The 5-year cumulative risk (CR) of contralateral breast cancer (CBC) was 0.55 for BRCA1/2, 0.89 for ATM, and 0.80 for CHEK2 carriers. At 10 years, overall CR increased to 0.65, with ATM and CHEK2 remaining high. Rate ratio (RR) analysis showed a significant risk for ATM (2.98), while overall RR indicated more than a two-fold increased CBC risk with radiotherapy (RR = 2.70 common-effect; 2.53 random-effects). Conclusions: Radiotherapy significantly increases contralateral breast cancer risk, particularly in ATM and CHEK2 carriers, emphasizing the importance of personalized genetic risk stratification in treatment decisions.