PURPOSE: To evaluate early cardiac single photon computed tomography (SPECT) findings after left breast/chest wall postoperative radiation therapy (RT) in the setting of deep inspiration breath hold (DIBH). METHODS AND MATERIALS: We performed a prospective single-institution single-arm study of patients who were planned for tangential RT with DIBH to the left breast/chest wall (± internal mammary nodes). The DIBH was done by use of a controlled surface monitoring technique (AlignRT, Vision RT Ltd, London, UK). The RT was given with tangential fields and a heart block. Radiation-induced cardiac perfusion and wall motion changes were assessed by pre-RT and 6-month post-RT SPECT scans. A cumulative SPECT summed-rest score was used to quantify perfusion in predefined left ventricle segments. The incidence of wall motion abnormalities was assessed in each of these same segments. RESULTS: A total of 20 patients with normal pre-RT scans were studied; their median age was 56 years (range, 39-72 years). Seven (35%) patients also received irradiation to the left internal mammary chain, and 5 (25%) received an additional RT field to supraclavicular nodes. The median heart dose was 94 cGy (range, 56-200 cGy), and the median V25 was zero (range, 0-0.1). None of the patients had post-RT perfusion or wall motion abnormalities. CONCLUSIONS: Our results suggest that DIBH and conformal cardiac blocking for patients receiving tangential RT for left-sided breast cancer is an effective means to avoid early RT-associated cardiac perfusion defects.