Background: As the number of long-term breast cancer survivors has increased, the side effects of adjuvant cancer therapy, such as cardiac toxicity, remain clinically important. Aims and objectives: To assess radiation-induced cardiotoxicity by estimating LVEF with 2D-ECHO and MUGA scan in left sided breast cancer patients treated with 2D RT vs 3DCRT technique. Material and methods: This a prospective study was conducted in the Department of Radiation Oncology at Guru Gobind Singh Medical College, Faridkot for a period of one year with a follow-up of 6 months started in February 2020, and included histologically proven patients with breast cancer. 50 patients fulfilling the inclusive and exclusive criteria were randomized into two arms (A and B) with 25 patients in each arm. Dosage was 50Gy/25#/5weeks Results: In both the ARM A and ARM B, there was RTOG grade I cardiac function toxicity post 6 months of RT that is LVEF decrease (<20%) from baseline. With 2D-Echo, the mean LVEF in ARM A before RT was 58.08, after 8 weeks of RT was 53.44 and after 6 months of RT was 52.04. With 2D-Echo, mean LVEF in ARM B before RT was 59.32, after 8 weeks of RT was 56.96, and after 6 months of RT was 55.48. LVEF with 2D-Echo was less in ARM A as compare to ARM B post 8 weeks and 6 months of RT with significant p value (0.001) Hence proving ARM A was more carditoxic as compared to ARM B. Whereas no significant difference was seen at baseline while comparing ARM A and ARM B. With Cardiac MUGA scan, mean LVEF in ARM A before RT was 67.03, after 8 weeks of RT was 62.31, and after 6 months of RT was 59.62. With the Cardiac MUGA scan, the mean LVEF in ARM B before RT was 66.54, after 8 weeks of RT was 63.87 and after 6 months of RT was 62.36. The difference in both arms was not statistically significant. On comparing of LVEF with 2D-ECHO mean LVEF before RT was 58.70, 8 weeks post-RT was 55.16 and 6 months of RT was 53.76 and with Cardiac MUGA scan mean LVEF before RT was 66.79, after 8 weeks of RT was 63.09 and 6 months of RT was 60.99 indicating the significant difference between 2D-ECHO and Cardiac MUGA scan with a p value of (0.001). Conclusion: Clinician must be aware that cardiotoxicity in left-sided breast cancer patients is a multifactorial issue, choosing and monitoring the type of radiotherapy according to patient condition is very important for decreasing late cardiotoxicity. New radiotherapy techniques using 3D- Conformal RT has resulted in minimal doses to the heart leading to declining cardiotoxicity and death.