[en] Particle radiotherapy was introduced to enhance the effect on cancers and to improve the dose distribution. Biologically, it is effective for hypoxic cancer cells and cancer cells under active recovery from radiation injury. The effect appears uniformly in all cell phases. This therapy is superior to the conventional radiotherapy for carcinoma of the head and neck, does not show great differences for carcinoma of the esophagus, lung or uterine cervix. This method shows an excellent effect on osteosarcoma. For malignant melanoma, reduction of the surgical area by preoperative irradiation is under study. The dose distributions of protons, ^-π mesons and heavy ions are much better than that of fast neutrons so that injury of normal tissues is remarkably reduced. The dose distribution of protons is best, and the method is appropriate for the treatment of malignant melanoma of the chorioidea. Although the therapeutic effect of high LET radiations is thought to be also demonstrated with heavy ions and ^-π mesons which show an excellent dose distribution and a characteristic biological effect, it requires the precise diagnosis of tumors and utilization and management of the equipment. (Chiba, N.)