Patients with left-sided breast cancer receiving radiotherapy are at risk of developing chronic cardiac toxicities. Cardiac dose-sparing techniques such as deep-inspiration breath-hold (DIBH) can reduce this risk. In this study, a model has been developed including tumor localization, left lung volume, and the distance between the heart and medial chest wall as well as age and Eastern Cooperative Oncology Group (ECOG) performance status, which may help to guide the decision for or against DIBH after planning computed tomography (CT) in free breathing directly before initiation of radiotherapy planning.