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size the importance of radiation therapy as a major treatment modality for cancers.Developments in radiation therapy since 1981When I joined this field in 1981, radiation therapy techniques were highly primitive. Virtually all insti‑tutions in Japan were equipped only with low-en‑ergy photon sources, such as cobalt-60 machines or low-energy medical linear accelerators (Linacs). Treatment was usually performed using a simple technique, such as two opposing anterior-posterior and posterior-anterior ports. The field was trimmed using one or two monoblocs fabricated from thick heavy metals. The radiation treatment field was determined using an X-ray simulator, which is a type of X-ray fluoroscopy specifically designed for radiation therapy treatment planning, and it has the same geometric arrangement as the treatment device. The field was determined based on anatom‑ical landmarks, such as bones, which are visualized using X-rays. For example, radiation fields for uterine cervical cancer were determined at the upper end, between the 4th and 5th lumbar verte‑brae, the lateral margin at 1.5 cm lateral to the inner margin of the iliac bone, and the lower margin at the level of the caudal end of the obturate foramen. However, some institutions did not have an X-ray simulator; as such, they had to use fluo‑Figure 1　A male patient with prostate cancer underwent salvage radiation therapy for biochemical recurrence after surgical resection. However, his prostate-specific antigen levels gradually increased after five years. Computed tomography (CT) could not detect any recurrent lesions (A); however, 18F- fluorodeoxyglucose positron emission tomography combined with CT clearly revealed suspected lymph node disease (B, indicated by white arrows). This lesion was treated using stereotactic radiation therapy using a total dose of 50 Gy in 10 fractions. Dose distributions of the treatments (C). Prostate-specific antigen levels dramatically decreased after treatment.roscopy dedicated to diagnostic use or simple X-ray photography. Low-energy photons cannot sufficiently penetrate to deep-seated areas of disease because they rapidly lose their energy along their track in the human body. Therefore, an extremely high dose was administered to the skin to treat deep-seated tumors, which sometimes caused severe side effects. The aforementioned factors, therefore, limited cure to only diseases located in superficial regions or easily approachable tumors, such as uterine cervical cancers in the early 1980s. As such, it was generally believed that radiation therapy was not a curative but a pallia‑tive method. However, rapid advances in computer science and mechanical technologies have led to a revolu‑tion in the field of radiation oncology. Linacs with ultra-high-energy X-rays, which can easily reach deep-seated lesions, are now commercially avail‑able. In the final decade of the 20th century, many new techniques emerged. If sufficient radiation doses could be delivered to the target volume, desirable performance in treatment was achieved. Stereotactic radiosurgery uses three-dimensional images and focuses multidirectional beams on a small target. The treatment was first applied to intracranial lesions, then gradually extended to extracranial diseases (Figure 1) and has yielded a 333