Radiotherapy is one of the three therapies along with surgery and chemotherapy that treats cancer. Besides cancer, the radiotherapy is also applied for treating some benign diseases. Radiotherapy is advantageous in that it is possible to achieve a survival rate similar to the radical surgery, while preserving organ functions, such as early breast cancer, glottic cancer, and anal cancer. Recently, radiotherapy is advancing rapidly to 3D conformal radiation therapy, Intensity Modulated Radiation Therapy, and image-guided radiation therapy along with the development and advancement of computers. The improvement reduces side effects while enhancing the therapeutic effect.