Purpose In left-side breast radiation therapy (RT), doses to the left main (LM) and left anterior descending (LAD) coronary arteries are usually assessed after delineation by prior anatomic knowledge on the treatment planning computed tomography (CT) scan. In this study, dose sensitivity due to interindividual coronary topology variation was assessed, and hot spots were located. Methods and Materials Twenty-two detailed heart models, created from heart computed tomography angiographies, were fitted into a single representative female thorax. Two breast RT protocols were then simulated into a treatment planning system the first protocol comprised tangential and tumoral bed beams (TGs-TB) at 50 + 16 Gy, the second protocol added internal mammary chain beams at 50 Gy to TGs-TB (TGs-TB-IMC). For the heart, the LAD, and the LM, several dose indicators were calculated dose-volume histograms, mean dose (D mean), minimal dose received by the most irradiated 2% of the volume (D2%), and 3-dimensional (3D) dose maps. Variations of these indicators with anatomies were studied. Results For the LM, the intermodel dispersion of Dmean and D2% was 10% and 11%, respectively, with TGs-TB and 40% and 80%, respectively, with TGs-TB-IMC. For the LAD, these dispersions were 19% (Dmean) and 49% (D2%) with TGs-TB and 35% (Dmean) and 76% (D2%) with TGs-TB-IMC. The 3D dose maps revealed that the internal mammary chain beams induced hot spots between 20 and 30 Gy on the LM and the proximal LAD for some coronary topologies. Without IMC beams, hot spots between 5 and 26 Gy are located on the middle and distal LAD. Conclusions Coronary dose distributions with hot spot location and dose level can change significantly depending on coronary topology, as highlighted by 3D coronary dose maps. In clinical practice, coronary imaging may be required for a relevant coronary dose assessment, especially in cases of internal mammary chain irradiation. © 2014 Elsevier Inc. All rights reserved.