Background: Radiotherapy (RT) is a major component of breast cancer treatment . However, breast RT is associated with long-term cardiac toxicity with clinical detection only relevant 10 to 15 years after RT. Early surrogate endpoints, predictive for the development of later cardiac events, would be useful. Purpose : To evaluate echocardiographic strain imaging for the detection of subclinical myocardial dysfunction arising after breast cancer RT. Methods: BACCARAT is a monocentric prospective cohort study of unilateral breast cancer patients treated with RT but without chemotherapy. They are followed for 2 years with repeated cardiac imaging examinations, including 2-dimensional speckle strain imaging for the detection of subclinical left ventricular abnormalities. Results : Between October 2015 and December 2017, this study prospectively included 114 women (95 left-sided and 19 right-sided) aged 58±8 years. Cardiac evaluations with strain imaging are performed before RT, 6 months and 24 months after RT. At this stage of the study, strain imaging measurements before RT and RT+6 months are available for 65 patients. Mean heart and left ventricle doses were 2.9 ± 1.3 Gy and 6.1 ± 2.9 Gy for the left-sided patients , 0.4 ± 0.1 Gy and 0.1 ± 0.04 Gy , respectively, for the right-sided patients. A decrease in global longitudinal strain was observed at 6 months post-RT for left-sided patients (-14.1% ±3.6% at 6 months after RT vs -15.4% ± 3.2% before RT, percentage of change 9.2%, P=0.02) but not for right-sided patients. No significant change was observed with conventional echocardiography left ventricular ejection fraction measurement. Conclusion : These preliminary results demonstrate a statistically significant reduction of global longitudinal strain 6 months after RT. Further investigation of BACCARAT study could illustrate strain imaging may have a potential role for screening and identification of patients at risk to develop cardiac complications.