Background Left-sided breast cancer radiotherapy poses a challenge in balancing target coverage with the sparing of critical cardiac substructures. A novel hybrid technique - RapidArc Dynamic (RAD) - integrated continuous arc delivery of volumetric modulated arc therapy (VMAT) with fixed-beam modulation of intensity modulated radiotherapy (IMRT) through strategic gantry pauses and dynamic collimator rotation.Purpose This study aims to evaluate the dosimetric performance of RAD, benchmarking it against IMRT and VMAT, with a specific focus on cardiac substructure sparing and normal tissue complication probability (NTCP).Methods Twenty patients with left-sided breast cancer treated with post-mastectomy adjuvant radiotherapy were retrospectively enrolled. For each patient, three plans (RAD, IMRT, and VMAT) were generated with a prescription of 50 Gy in 25 fractions. RAD plans utilized a “balanced” optimization strategy combining static ports and dynamic arcs with strategic gantry pauses to enhance modulation. Dosimetric metrics were analyzed for the planning target volume (PTV) and organs at risk (OARs). NTCP values were calculated using the Lyman-Kutcher-Burman (LKB) model for multiple endpoints.Results RAD demonstrated significantly lower dose metrics for the heart, left lung, and spinal cord compared to both IMRT and VMAT (P < 0.05). Specifically, RAD achieved superior sparing of the left anterior descending coronary artery (LAD) and left ventricle (LV), reducing mean and maximum doses significantly compared to competing modalities. While VMAT provided better conformity than IMRT, it resulted in a higher low-dose bath to the contralateral breast. Radiobiological modelling demonstrated that RAD successfully mitigated this issue, delivering significantly lower contralateral doses than VMAT. RAD yielded significantly lower predicted NTCPs for cardiac perfusion defects, LA major adverse cardiac events, and pulmonary pneumonitis compared to IMRT and VMAT.Conclusion RAD offers a feasible and effective alternative for left-sided chest radiotherapy, providing an optimal balance between high dose conformity and superior organ sparing.