Background: to assess the benefit of a deep inspiration breath hold (DIBH) over standard irradiation technique, and eventually to identify anatomical and/or treatment preplanning characteristics correlated with LAD dose. Methods: We retrospectively identified patients with left-sided breast cancer who underwent whole breast radiotherapy in DIBH. All patients had both plans in DIBH and free-breathing (FB). Receiving operating characteristics (ROC analysis) were performed to define the cut-off point of parameters to predict LAD maximum dose >10 Gy and LAD mean dose > 4 Gy. Areas under the curve (AUCs) were calculated for all variables. Post-test probability has been calculated to evaluate advantage for parameters combination. Results: One hundred ninety-seven patients were identified. LAD dose was significantly reduced in DIBH plans with maximum and mean dose reduced by 31.7% (mean value 3.5 Gy vs 4.8 Gy, p=<0.001) and 28.1% (mean value 8.2 Gy vs 12.8 Gy, p=<0.001) in DIBH plans compared to FB plans. The strongest predictor of LAD maximum dose > 10 Gy and a LAD mean dose > 4 Gy was the minimum distance of LAD from tangent open fields. Other parameters were lung volume and heart volume for LAD Dmax > 10 Gy and lung volume, heart volume and breast separation for LAD Dmean > 4 Gy. Conclusion: The dosimetric benefit of DIBH is valid for all patients and DIBH should be preferred for all left sided patients.