To compare the dosimetry of a large clinical series of left-sided breast cancer patients treated with forward intensity modulated radiation therapy in free breathing (FB) or in breath hold (BH).A total of 155 early-stage pT1c-T2N1 left breast carcinoma patients receiving whole breast irradiation were enrolled in this study. Computed tomography scans acquired in FB (75 patients) and in BH (82 patients) were exported to the treatment planning. Clinical target volume (CTV) and organs at risk (OARs) were contoured by an experienced radiation oncologist of the breast department; the planning target volume (PTV) was generated with an isotropic expansion of the CTV with a 3 mm margin in all directions, excluding the first 5 mm inside the body external contour. Patients were treated with a prescribed dose (Dp) of 50 Gy in 25 fractions. All plans were based on a tangential field in field technique with a 6 MV photon beam energy using two opposing tangential fields, each including 3 to 5 sub-segments. Treatment plans were optimized to achieve minimum 95% of the PTV covered by 95% isodose line and a mean dose () to the PTV equal to the Dp and keeping the organs at risk (OARs) doses as low as achievable. Dose volume histogram (DVH) endpoints for the PTV (D95, D98, Dm, Dmax), for heart and left lung (V5, V20, Dm) and for right lung and right breast (Dm and Dmax) were used to compare the dosimetry of FB and BH treatments. Data were analyzed with the Wilcoxon Cox test; P value < 0.05 was considered statistically significant.The analysis showed no significant difference for PTV D95, D98, Dm, Dmax, and volume (P = 0.329, P = 0.456, P = 0.566, P = 0.067, P = 0.482, respectively) between BH and FB groups. Comparing the dosimetry of OARs, we recorded significantly lower values for patients in the BH group: heart Dm (1.7 Gy vs 5.3 Gy), V5 (3.7% vs 26.0%) and V20 (0.4% vs 3.0%); left lung V5 (29.5% vs 38.0%) and V20 (13.6% vs 15.7%); right lung Dm (0.1% vs 2.5%), right breast Dm (0.2 Gy vs 1.7 Gy). While, a slight lower lung left Dm value (8.0 Gy vs 8.6 Gy), not statistically significant (P = 0.106) was registered for BH group. The OARs dosimetry obtained for the FB patients and the corresponding percentage difference obtained for the BH group is shown in the Table below.The use of BH in the clinical practice may be the best option for left breast RT reducing the irradiation of heart, left lung, and of right lung and right breast, without compromising target coverage.Copyright © 2021. Published by Elsevier Inc.