Objective: The objective of this study was to prospectively collect and report treatment planning data in terms of organs at risk (OARs) sparing effect between deep inspiration breath hold (DIBH) and free breathing (FB) computed tomography (CT) scans. This study also aims to identify potential planning parameters that could help in selecting patients most likely to benefit from DIBH. Methods: Thirty-eight patients with left-sided breast malignancy indicated for adjuvant radiotherapy underwent DIBH and FB CT simulation. All patients were planned with a three-dimensional conformal radiation therapy (3D-CRT) for both scans. Comparisons of dosimetric variables include heart Dmean, left anterior descending coronary artery (LAD) Dmean/Dmax, left lung V30Gy, V20Gy, V10Gy, V5Gy, FB axial cardiac contact distance (FB-CCDax) and parasagittal CCD (FB-CCDps). Results: DIBH resulted in a statistically significant reduction of heart Dmean, LAD Dmean and Dmax. When DIBH was compared with FB, heart Dmean was 1.62 Gy versus 2.65 Gy; for LAD Dmean, 6.81 Gy versus 11.57 Gy; and for LAD Dmax, 22.66 Gy versus 31.93 Gy. Left lung dosimetry results consistently showed all the volume parameters of V5Gy to V30Gy for FB were significantly higher than that of DIBH. There was a significant positive correlation between FB-CCDax/FB-CCDps and mean heart absolute dose reduction. A meaningful positive correlation was observed for FB-CCDps beyond the cutoff length of 2cm. Conclusion Our study has confirmed the benefit of DIBH in reducing mean heart and lung dose in left-sided breast/chest wall radiotherapy. FB-CCDps is a potentially reliable parameter to guide us in selecting patients whom would benefit most for DIBH.