Since the discovery of X-rays by Röntgen in 1895, X-rays have been used to treat a wide variety of diseases including malignant tumors. The primary principle of radiotherapy (RT) lies in sufficient and precise dose localization in the target while minimizing a damage of the surrounding normal tissues. In this sense, the era of 1950s marked the beginning of modern RT, when high-energy machines such as a tele-cobalt and linear accelerator were developed and introduced in clinical practice. In the late 20th century, computer technology allowed the development of high-technology treatment such as intensity modulated RT (IMRT) and stereotactic body radiotherapy (SBRT), which significantly contributed to improvement of treatment results. IMRT delivers photon beams at the target volume from many different directions, thereby permitting high dose concentration in the target while diluting the dose outside the treatment volume. Particle RT has a history of more than 70 years and has enhanced the clinical potentials of RT. Among various types of ion species, particular interest has been focused on protons and carbon-ions. This is based on the fact that, when compared to photons, they share providing beneficial dose distribution. Furthermore, in the case of carbon ions, they possess a large biological effectiveness (RBE), and higher probability of tumor control with limited damage to the surrounding normal tissues can be expected. Currently, there are more than 50 facilities in the world for proton and carbon-ion therapy and still more facilities are under construction or active planning.
