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Introduction: Radiation therapy to left breast cancer patients results in significant exposure to heart resulting in long-term cardiac morbidity. This exposure can be reduced by performing deep inspiratory breath-hold (DIBH) technique, however, patient selection criteria or predictive parameters are not routinely used which can identify patients who will have significant benefit with this technique. This study intends to find and use these predictive parameters measured on free-breathing (FB) simulation scan which can help to select such patients so as to develop our institutional protocol. Methods: A total of 35 patients of left breast cancer postsurgery were recruited in the study. All patients underwent 2--3 sessions of DIBH coaching followed by planning computed tomography scan in both FB phase and DIBH phase. Various anatomical parameters such as cardiac contact distance parasagittal (CCDps), CCD axial, heart--chest distance (HCD), and D were measured on FB scan and were correlated with a mean heart dose achieved in DIBH scan. Results: Dose to normal structures, namely heart and lung, was drastically reduced for DIBH scan over FB scan. V5, V10, and V20 of heart also followed a similar reduction. The mean dose to heart in FB scan was 5.34 Gy, while in DIBH scan, it was 2.79 Gy which was statistically significant. Among all the parameters measured, CCDps and D had a statistically significant positive correlation with the mean heart dose in DIBH phase while HCD had a negative correlation which was statistically significant. Conclusion: Cardiac parameters on FB scan such as CCDps, HCD, and D predict the patients with usual cardiac risk who will benefit most with DIBH techniques setting an arbitrarily cutoff value. [ABSTRACT FROM AUTHOR]