Objective: Lymphatic irradiation in breast cancer improves locoregional control and has been shown to decrease distant metastasis. However, irradiation also accelerates the formation of atherosclerosis and can cause stenosis, not only in the coronary arteries but also in the internal mammary artery (IMA). The aim of this study was to investigate the effects of radiation on IMAs via computed tomography (CT). Methods: We reviewed the data of 3,612 patients with breast cancer treated with radiotherapy (RT) between January 2010 and December 2016. We included 239 patients with appropriate imaging and nodal irradiation in the study. All patients were treated with lymphatic irradiation of 46-50 Gy, and a boost dose (6-10 Gy) to the involved internal mammary nodes (IMNs) when imaging studies demonstrated pathological enlargement. Bilateral IMA diameter and the presence of calcification were assessed via thin contrast-enhanced CT and those of ipsilateral irradiated IMAs were compared with those of contralateral nonirradiated IMAs. Results: The mean diameter ofirradiated IMAs was significantly shorter than that of nonirradiated IMAs, regardless of laterality. All vascular calcifications were determined on the irradiated side. A boost dose of radiation to the IMNs and radiation technique did not significantly affect the IMA diameter or the presence of calcification. Conclusions: The diameter of the IMA is decreased due to RT regardless of laterality, radiation technique, and administration of a boost dose. Evaluation of vessels on CT before coronary artery bypass graft or flap reconstruction can help the surgeon select the most appropriate vessel.