The aim of this thesis will be to assess the risk of medium and long-term cardiotoxicity in patients treated with radiotherapy (RT) for breast cancer. The study of medium-term risk of cardiotoxicity will be based on the prospective monocenter BACCARAT study (BreAst Cancer and CArdiotoxicity induced by RAdioTherapy) coordinated by IRSN, which included around 100 patients followed for 2 years after RT with repeated echocardiogrphies and computed tomography coronary angiograms, and will allow 1 / to assess functional cardiac damage occurring or persisting 2 years after RT as well as coronary damage occurring or progressing 2 years after RT, and their links with the dose absorbed by heart during RT; 2 / to identify relevant plasma biomarkers in the evaluation of these functional and coronary damages. The study of long-term cardiotoxicity will be performed using data from the prospective multicenter CANTO (CANcer Toxicities) cohort coordinated by UniCancer, consisting of more than 12,000 patients followed for at least 5 years, and will make it possible to assess the risk of cardiac pathologies (Heart failure, Acute coronary syndrome, ischemic heart disease, myocardial infarction) and the relationship with the dose absorbed by the heart during RT. ​