BACKGROUND: Radiotherapy for left-sided breast cancer is associated with additional cardiac risks. Deep inspiration breath hold (DIBH) has emerged as the primary cardiac sparing technique, demonstrating significant reduction in mean heart dose (MHD) and left anterior descending coronary artery (LAD) dose. However, the issue of whether dosimetric advantages can be effectively translated into clinically measurable benefits for patients remains to be further elucidated. The study aims to investigate whether DIBH, in contrast to free breathing (FB), could yield a clinical cardiac advantage, beyond dosimetry benefits, for patients with early left breast cancer following breast-conserving surgery combined with whole breast radiotherapy. PATIENTS AND METHODS: The study involved 78 patients with early-stage left breast cancer undergoing radiotherapy between 2021 and 2022 after breast-conserving surgery. 46 patients were treated with DIBH technique and 32 were treated with FB. Patients with previous cardiac disease were excluded. We performed myocardial enzymes, electrocardiogram (EKG), and echocardiograms (ECHO) in all patients within 2 weeks before, during, and 6 months after radiotherapy. EKG and ECHO follow-up extended to 24 months. The results of the two groups were compared using nonparametric tests and chi-square tests, and P <
0.05 indicated statistical significance. Subclinical acute cardiac injury was characterized by the emergence of elevated myocardial enzymes beyond normal levels and/or EKG alterations in ST-T or T-wave patterns and/or abnormalities detected in ECHO within a 6-month period following radiotherapy. RESULT: The average age was 52.3 years for FB and 44.9 years for DIBH. There were no significant differences in staging, molecular subtype, chemotherapy, and endocrine therapy history. The proportion of subclinical acute cardiac injury was lower with DIBH compared to FB (DIBH = 31/46 and FB = 28/32, p = 0.042). The most sensitive subclinical acute cardiac injury events were detected by myocardial enzymes, with cardiac troponin I (cTnI, p = 0.034) and N-terminal pro-B-type natriuretic peptide (NT-proBNP, p = 0.023) appearing significantly lower in the DIBH patients during radiotherapy. The difference of cTnI between 2 groups at 6 months after radiotherapy became non-significant. Creatine kinase-MB (CK-MB) was higher in DIBH comparedwith FB only 6 months after radiotherapy (p = 0.006). The differences in EKG and ECHO were not significant between the two groups within 6 months after radiotherapy. Notably, the DIBH group exhibited a significantly lower incidence of abnormal EKG at 12-24 months compared to the 3-6 months (p = 0.003). CONCLUSION: The implementation of DIBH has been demonstrated to reduce the proportion of patients experiencing subclinical acute cardiac injury, with a higher proportion of patients returning to normalcy after one year. This finding suggests the potential reversibility of radiation-induced cardiac injury. The monitoring of these early cardiac biomarker changes to identify high-risk patients, followed by the implementation of early interventions, may contribute to a reduction in the incidence of long-term cardiac complications. Future research could integrate cardiac biomarkers, EKG, as well as traditional dosimetric parameters and baseline cardiac risk factors, to construct a more precise, individualized cardiac risk prediction model, thereby better assessing the long-term cardiac risks in patients.