**1. Introduction**

Cancer is a leading cause of death in both developed and less developed countries worldwide, and its health burden is expected to increase rapidly [1]. In 2012, an estimated 14.1 million new cancer cases and 8.2 million deaths occurred worldwide [1]. Currently, approximately 57% of cancer cases and 65% of cancer deaths occur in less developed countries [1]. Worldwide, the new cases or deaths from lung and breast cancer were at the top of the list [1]. In China, in 2015, an estimated 4,292,000 new cancer cases and 2,814,000 cancer deaths

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occurred [2]. Lung cancer is the most common incident cancer and the leading cause of cancer death in China, and esophageal cancer is also commonly diagnosed. Worldwide, lung, esophageal, and breast cancer account for approximately 27% of new cancer cases which means that more than 20% of patients will receive thoracic radiation therapy (RT). Many studies have proven that local RT improves local control and prolongs overall survival [3–11]. However, thoracic RT might inadvertently result in various forms of cardiac toxicity and manifest as clinical and subclinical cardiac disease, termed radiation-related heart disease (RRHD) [12, 13]. In this chapter, we will present the epidemiological data and discuss the possible pathophysiological mechanisms in brief. We will also address the cardiac avoidance techniques and the dose-volume-effect relationship. Although many cytotoxic and molecularly targeted drugs also result in various cardiac toxicities [14], consideration of these is outside the scope of this chapter.
