Background: Radiotherapy following Breast-conservative surgery is a standard treatment for breast cancer, Techniques developed to obtain better target definition, dose homogeneity and coverage, however, studies of radiation toxicity in the treatment of breast cancer showed that the effects on normal tissues can constitute a significant clinical problem and increased cardiac mortality in particular may offset survival benefit of treatment. Objectives: This a cross section planning study aiming at evaluation of brachial plexus, coronary artery & thyroid gland doses in previously treated breast cancer cases at Kasr Alaini Center of Clinical Oncology & Nuclear Medicine (NEMROCK) for post-operative loco-regional conformal irradiation after left Breast Conservative Surgery (BCS) with no previous care about brachial plexus, coronary artery or thyroid gland as our technique does not involve their routine delineation and to identify the patients group in need for routine delineation of these ROs to avoid toxic doses to these ROs. Methods: 25 patients previously treated at NEMROCK for post-operative loco-regional conformal irradiation after left BCS will be included in the study. Delineation of coronaries of the heart was done using the steps provided by University of Michigan Medical Center; while the brachial plexus was delineated according to the RTOG & Thyroid gland was delineated manually by its gross appearance. Results: Mid beam cut separation ranged from 16.3 cm to 28.7 cm with mean value ± SD (20.5 ± 3.0) while Central long distance ranged from 1.1cm to 4.0 cm with mean value ± SD (2.4 ± 0.7) . Maximum heart distance mean value ± SD was 2.9 cm ± 1.1 cm. Conformity index mean value was 1.71 with 0.12 SD. The mean value of breast V45 was 82.12 % with 21.3 % SD. While the mean value of the heart V30 was 3.44 % with 3.59 % SD. And the mean value of the heart Dmean was 3.92 Gy ± 2.02 Gy SD. The mean value of the CA Dmax was 41.9 Gy with 6.60 Gy SD. The mean value of the CA Dmean was 23.4 Gy with 10.9 Gy SD. The mean value of thyroid V40 was 32.7 % with 4.1 % SD while the mean value of thyroid Dmean was 20.6 Gy with 5.3 Gy SD. The brachial plexus Dmax was 46.7 Gy with 3.0 Gy SD while the mean value of the brachial plexus Dmean was 33.7 Gy with 6.4 Gy SD. The maximum heart distance when it was less than 3 cm, the mean of the Dmean of the anterior descending coronary artery was 18.5 Gy with SD 10.9 Gy while when more than 3 cm the mean of the Dmean of the anterior descending coronary artery was 27.9 Gy with SD 9.1 Gy with a P-value 0.030 which is statistically significant. The heart V30 when it was less than 2% the mean of the Dmean of the anterior descending coronary artery was 16.9 Gy with SD 10.5 Gy while when more than 2% it was 29.5 Gy with SD 7.3 Gy with a P-value 0.005 which is statistically significant. Conclusion: A significant dose may be received by non-routinely delineated organs at risk (of brachial plexus, coronary artery & thyroid gland) in post-operative loco regional radiotherapy of patients with left breast cancer after BCS. A significantly higher dose was received by left ADCA in cases with high MHD & heart V30 & borderline significance in obese cases where obesity is a known risk factor for developing coronary artery diseases.