The risk of RT-related heart disease is now well recognized but the underlying mechanisms of its initiation and progression, and the roles played by microvascular damage, fibrosis and atherosclerosis remain unclear. Studying RT-related heart disease have demonstrated that irradiation affects cardiac structure and microvascular function in a dose and time-dependent manner, with substantial damage after intermediate and high dose irradiation, and minor alterations after lower doses. Not only structural changes but also functional deterioration was noted. Studies disclosing that incidence of post 3D-RT perfusion defect is in approximately 40%, and the patients had volume-dependent perfusion defects within 2 years of RT.