Postoperative radiotherapy in the management of breast cancer has been proven to be effective in reducing local and distant recurrence but survival advantage may be limited by increased non-cancer related mortality which may be attributed to radiation doses received by risk structures. Three- dimensional conformal radiotherapy (3D-CRT) may reduce the unwanted irradiation and consequently may improve quality of life and/or survival. This work aim to compare between 2 dimensional (2D) and 3D-CRT in the postoperative management of left breast cancer considering homogeneity of the dose distribution inside the target volume(s) and doses received by the surrounding risk structures specially the heart and left lung. Clinical outcome included tumor control, survival and toxicity for both techniques. Sixty female patients with left breast cancer following mastectomy or breast conservative surgery were included and each one had both 2D and 3D conformal planning. Both techniques were compared (planning study) for target volume coverage, dose homogeneity, doses received by the heart and left lung, and the estimated excess relative risk of right breast cancer. As for treatment (clinical study), only one planning technique was randomly applied and accordingly patients were divided into group A including 30 patients received treatment based on 2D planning and group B including 30 patients received 3D-CRT. The planning study showed that there was no statistically significant difference between the 2 techniques in coverage and dose homogeneity inside the left breast (following conservative surgery), chest wall (following mastectomy) or supraclavicular lymph nodes (SCLN). However, better coverage was demonstrated inside the internal mammary nodes (IMN) in favor of the 3D conformal plan. The volume received 45 Gy (V45Gy) was 76.47±20.195 % for 2D vs 89±8.856 % for 3D (P-value =0.036). Another important result was sparing the left lung from receiving a dose of 20 Gy or more (V20Gy) in favor of the 3D conformal plan. The estimated excess relative risk of right breast cancer was less in the 3DCRT (1.68 ± 0.815 %) compared to the 2D (2 ± 0.66 %), but the difference was not statistically significant (P-value = 0.599). In the clinical study, after a median follow up period of 29.5 months, there was no significant difference between both arms in the locoregioal recurrence, survival as well as the toxicity. Only small but statistically significant difference was noticed between both arms in the degree of reduction of cardiac Ejection Fraction (EF) as measured by isotopic scanning in favor of the 3D conformal therapy (5.127±4.8839% for 2D vs 2.363±4.7562% - P-value = 0.013). No cosmetic difference was noticed between both groups at the end of the study. The study concluded that 3D-CRT spared the left lung from receiving higher radiation dose during the post-operative radiotherapy in the management of left breast cancer with only small but significant less reduction of cardiac EF. 3D-CRT should be offered for patients who are going to receive IMN irradiation for better coverage of the target volume.