Purpose: To quantify the cardiac dose reduction during breathing adapted radiotherapy using Real-time Position Management (RPM) system in the treatment of left-sided breast cancer. Materials and Methods: Twenty-two patients with left-sided breast cancer underwent CT scans during breathing maneuvers including free breathing (FB), deep inspiration breath-hold (DIBH), and end inspiration breath-hold (EIBH). The RPM system was used to monitor respiratory motion, and the in-house self respiration monitoring (SRM) system was used for visual feedback. For each scan, treatment plans were generated and dosimetric parameters from DIBH and EIBH plans were compared to those of FB plans. Results: All patients completed CT scans with different breathing maneuvers. When compared with FB plans, DIBH plans demonstrated significant reductions in irradiated heart volume and the heart $V_{25}$, with the relative reduction of 71% and 70%, respectively (p < 0.001). EIBH plans also resulted in significantly smaller irradiated heart volume and lower heart $V_{25}$ than FB plans, with the relative reduction of 39% and 37%, respectively (p = 0.002). Despite of significant expansion of lung volume using inspiration breath-hold, there were no significant differences in left lung $V_{25}$ among the three plans. Conclusion: In comparison with FB, both DIBH and EIBH plans demonstrated a significant reduction of radiation dose to the heart. In the training course, SRM system was useful and effective in terms of positional reproducibility and patient compliance.