After its discovery by Wilhelm Roentgen in 1895, radiation started to be used as a diagnostic and therapeutic modality for cancer (Table 2.1). A patient with superficial breast cancer was treated with kilovoltage (kV)- range radiotherapy (RT) in 1896, but it was not until 1951 when the first cancer patient was treated with Cobalt-60 radiation. In 1956 when megavoltage (MV) energy-based linac was developed at Stanford University, it became possible to treat deep tumors. As a result of the development of computed tomography (CT) technology in the early 1970s, imaging techniques for cancer advanced significantly from 1970 to 1980. Finally, three-dimensional (3D) CT imaging and RT planning were widely implemented in the 1990s, revolutionizing RT.