Objective: To investigate the dose-protective effects on cardiac substructures of Tomotherapy (TOMO) versus Intensity-Modulated Radiation Therapy (IMRT) with fixed fields during large-field radiotherapy following breast-conserving surgery for left-sided breast cancer, and to compare their short-term efficacy differences. Methods: A retrospective analysis of clinical data from 86 patients who underwent large-field radiotherapy after breast-conserving surgery for left-sided breast cancer. Patients were categorized by treatment modality into the TOMO group (n=43) and the IMRT group (n=43). The postoperative changes in the tumor bed region, target volume and organ at risk dose parameters, echocardiographic indicators, and the incidence of adverse reactions were compared between the two groups. Results: At 6 months post-radiotherapy, the TOMO group had a higher V95% coverage of the target volume than the IMRT group, a higher Conformity Index (CI) closer to 1, and a lower Homogeneity Index (HI) than the IMRT group (P<0.05). In the TOMO group, the dosimetric parameters for the heart and left ventricle [maximum dose (Dmax), mean dose (Dmean), V5Gy, and V20Gy] were all higher than those in the IMRT group, and the V30Gy for the heart was higher than that in the IMRT group (P<0.05). In the TOMO group, the Dmax, Dmean, and V20Gy of the left anterior descending artery (LAD) were lower than those in the IMRT group (P<0.05). In the TOMO group, the Dmean and V20Gy of the ipsilateral lung were lower than those in the IMRT group (P<0.05), while the Dmean and V5Gy of the contralateral lung and contralateral breast were higher than those in the IMRT group (P<0.05). The Dmean and V20Gy of the humeral head (affected side) were lower than those in the IMRT group (P< 0.05). The LVEF level in the TOMO group was higher than that in the IMRT group (P< 0.05). There was no statistically significant difference in the incidence of adverse reactions between the two groups (P>0.05). Conclusion: Compared with IMRT, TOMO delivers superior dose quality to the target volume in high-dose fractionation radiotherapy for left-sided breast cancer. It reduces doses to the left anterior descending artery, ipsilateral lung, and humeral head, but also increases overall cardiac dose and dose to contralateral structures to some extent.