Simple SummaryRadiation-induced heart disease represents a spectrum of early and late effects. It is a significant concern for cancer survivors who receive thoracic radiation therapy, as it is the most common nonmalignant cause of death in this population. Subclinical cardiac damage can be detected at an early stage after irradiation and may possibly be used to predict late cardiac complications. Modern radiotherapy techniques, such as deep inspiration breath-hold radiotherapy or intensity-modulated radiation therapy, can reduce the heart dose and probably do not cause alterations in myocardial perfusion. This paper describes the study protocol we will use to evaluate perfusion defects after left-sided breast radiotherapy using myocardial perfusion single-photon computed tomography.AbstractBreast radiotherapy can lead to radiation-induced cardiac disease, particularly in left breast cancers. Recent studies have shown that subclinical cardiac lesions, such as myocardial perfusion deficits, may occur early after radiotherapy. The primary method for irradiating breast cancer, known as opposite tangential field radiotherapy, can cause the anterior interventricular coronary artery to receive a high dose of radiation during left breast irradiation. To explore alternative approaches that could reduce the risk of myocardial perfusion defects in patients with left breast cancer, we plan to conduct a prospective single-center study using a combination of deep inspiration breath hold radiotherapy and intensity modulated radiation therapy. The study will use stress and, if necessary, resting myocardial scintigraphy to assess myocardial perfusion. The trial aims to show that reducing the cardiac dose with these techniques can prevent the appearance of early (3-month) and medium-term (6- and 12-month) perfusion disorders.