Broeks et al.32 used a case-only design to assess the interaction between radiotherapy (RT) for treatment of a first breast cancer and mutations in four DNA damage repair genes (BRCA1, BRCA2, CHEK2, and ATM) on the subsequent risk of contralateral breast cancer (CBC). Among RT+ cases, there was a 2.2-fold higher prevalence of germline mutations in one or more of these genes than among RT– cases. Here it seems unlikely that genotypes would have affected the choice of treatment, except perhaps indirectly through tumor characteristics or stage at diagnosis (factors that could be adjusted for).
