Background: Protecting cardiac function in patients with advanced left-breast cancer receiving radiation therapy (RT) with regional nodal irradiation (RNI) is an important issue. Modern RT techniques can limit cardiac exposure. The aim of this study was to explore the association be-tween cardiac dose and cardiac function. Methods: Between 2017 and 2020, we retrospectively reviewed left-breast cancer patients who received adjuvant RT, including RNI with either volumetric-modulated arc therapy (VMAT) or helical tomotherapy (HT). Left ventricular ejection fraction (LVEF) was assessed by echocardiography before RT and 1 year after RT to detect any early deterioration in cardiac systolic function. Results: A total of 30 eligible patients were enrolled. The median follow-up time from the initiation of RT was 3.9 years (range 0.6–5 years). Seventeen patients received VMAT, and the other 13 patients received HT. The median RT dose was 55 Gray (Gy), and the mean heart dose was 3.73 Gy (range 1.95–9.36 Gy). The median LVEF before and after RT was 68% and 68.5%, respectively. No obvious deterioration was found. There was no association between cardiac dose (mean heart dose, V5–V30) and LVEF (change in values or post-RT). Conclusions: For left-breast cancer patients undergoing RT with RNI, VMAT, or HT can be used to limit cardiac exposure. Cardiac function as evaluated by LVEF revealed no obvious deterioration after RT in our patients, and no association was found between cardiac dose and LVEF in those treated with either VMAT or HT in early cardiac surveillance.