Background and purpose <p>Radiotherapy plays a pivotal role in breast cancer treatment. Incidental irradiation of the heart can cause cardiac toxicity. We conducted a dosimetric comparison between Hybrid VMAT (H-VMAT) and 3D Conformal Radiotherapy (3DCRT) in both Free Breathing (FB) and Deep inspiration breath hold (DIBH) techniques for left-sided breast cancer patients.</p> Materials and methods <p>Thirty-seven patients underwent CT scans in both FB and DIBH positions. Heart substructures were delineated following established guidelines. Subsequently, 3DCRT and H-VMAT plans were generated for each patient in both breath techniques. The dosimetric parameters of heart and its cavities were analysed: D_mean(Gy) and V5Gy(%) for heart substructures, D_mean(Gy), V25Gy(%) and V30Gy(cm³) for heart and D98%(%), D2%(%), D50%(%) and V95%(%) or V90%(%) for PTVs. Statistical analyses were performed.</p> Results <p>The analysis revealed statistically significant differences for the heart, and its cavities. The 3DCRT plans generated in DIBH offered a statistically significant lower dose for the heart and its substructures compared to the other techniques. When comparing PTV dosimetry, the H-VMAT plans showed an increase D98%(%) and a decrease D2%(%) relative to the 3DCRT plans, for both breathing techniques employed.</p> Conclusion <p>Our study demonstrates significant differences in the dosimetric outcomes for the heart substructures among the four evaluated techniques, with 3DCRT in DIBH yielding the lowest parameters for most substructures. Although H-VMAT provided superior target coverage, it resulted in greater low doses incidental exposure of the heart substructures. Notably, 3DCRT plans in the DIBH setting exhibited lower doses compared to H-VMAT in FB, supporting its preferential use for minimising cardiac exposure.</p>