Ionizing radiation kills tumors but may also damage normal tissues. The primary goal of radiation therapy is to deliver a lethal dose to the tumor tissues while sparing normal tissues. Efforts have been constantly made by the scientific community to maximize the therapeutic roles of radiation and to minimize its detrimental effects ever since the beginning of radiation oncology. Technology improvements in imaging and beam delivery have been changing the ways of cancer diagnosis and treatment in the clinic. The past two decades have witnessed the transition of modern radiotherapy from three-dimensional conformal radiation therapy (3D-CRT) to more advanced and complicated techniques such as intensity-modulated radiotherapy (IMRT), image-guided radiotherapy (IGRT), stereotactic body radiation therapy (SBRT), 124and volumetric modulated arc therapy (VMAT) [1–5]. Advanced beam delivery techniques have greatly improved dose conformity, making dose escalation possible for localized tumors. With the use of higher-than-conventional radiation doses, local and regional tumor control has been significantly improved over the years, although increased cases of late toxicity in normal tissues have been reported [6,7].