Background: Radiotherapy (RT) for left breast cancer poses a significant hazard of cardiac damage, despite advancements in three-dimensional conformal radiotherapy (3D-CRT). Identifying correlations between dosimetric parameters and markers of cardiac dysfunction is critical for optimizing treatment planning. Objectives: This study aimed to evaluate the impact of 3D-CRT boost techniques on cardiac toxicity, focusing on correlations between dosimetric parameters, cardiac biomarker and echocardiographic measurements. Patients and methods: We conducted a prospective study involving 50 females with left breast cancer receiving postoperative RT at Qena University Hospital. Dosimetric parameters, including mean cardiac dose and LV volume exposed to ≥ 30 Gy (V30) were calculated. Biomarkers [Brain Natriuretic Peptide (BNP)] and echocardiographic markers [Ejection Fraction (EF) and Global Longitudinal Strain (GLS)] were analyzed pre-radiotherapy, and at one and six months post-radiotherapy. Results: The average cardiac dose was 6.1 ± 3.2 Gy, and the average V30 was 8.2 ± 3.4%. BNP levels increased significantly post-RT (p < 0.001), while EF and GLS showed marked declines (p < 0.05). Significant statistical correlations were observed between V30 and GLS (r = -0.4490, p = 0.0011). Conclusion: RT for left-sided breast cancer increases cardiac toxicity. Correlations between V30 and cardiac dysfunction underscore the need for enhanced dose-sparing techniques to minimize long-term risks. Future strategies should integrate advanced modalities like deep inspiration breath-hold (DIBH) and proton therapy.