<p>Radiation-induced heart disease is a well-recognized late complication of thoracic radiotherapy and comprises a broad spectrum of cardiac disorders. Because myocardial metabolic impairment may precede overt structural or perfusion abnormalities, early or subclinical myocardial injury is often challenging to detect using conventional cardiac imaging modalities. We report a case of late-onset radiation-induced myocardial metabolic impairment identified by dual myocardial scintigraphy many years after thoracic radiotherapy for thymoma. The patient, a man diagnosed with malignant thymoma at 42 years of age, underwent two courses of thoracic radiotherapy during the course of his illness. The first course was confined to the mediastinum without cardiac exposure, whereas the second course, delivered at 52 years of age for recurrent disease with pericardial and myocardial invasion, resulted in direct cardiac irradiation. After a prolonged asymptomatic period, at 67 years of age (approximately 15 years after the second course of radiotherapy), the patient developed symptoms consistent with heart failure. Coronary angiography revealed no significant coronary artery stenosis. Dual myocardial scintigraphy demonstrated a characteristic metabolic–perfusion mismatch: reduced uptake of ¹²³I-β-methyl-p-iodophenyl-pentadecanoic acid (BMIPP) in the anterior to lateral wall of the left ventricle, precisely corresponding to the irradiated cardiac region, while myocardial perfusion assessed by ⁹⁹ᵐTc-methoxyisobutylisonitrile (MIBI) was preserved. Retrospective dosimetric analysis demonstrated a mean heart dose of 12.8&#xa0;Gy during the second course of radiotherapy, supporting a radiation-induced mechanism of myocardial impairment. This case highlights the clinical value of combined ¹²³I-BMIPP and ⁹⁹ᵐTc-MIBI myocardial scintigraphy for the noninvasive detection of late-onset radiation-induced myocardial impairment in long-term cancer survivors, even in the absence of obstructive coronary artery disease.</p>