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334very high frequency of disease eradication. From Juntendo University, Naoi and colleagues were pioneers in this field in Japan and published high-quality reports2, 3). Takahashi and colleagues proposed “conforma‑tion radiation therapy” in the 1960s4, 5). This is a type of rotational radiation therapy, in which the tumor(s) is irradiated in a 360°direction, and the beams are trimmed to conform to the shape of the target volume during irradiation. However, the technique was not very popular until the 1990s because it was very complicated and there was no way to obtain trans-axial images of the body except by using Takahashi’s rotation tomograms. In the 1990s, major instrument manufacturers equipped their Linacs with a multi-leaf collimator (Figure 2), which can easily shape the radiation field to conform to the target. Computed tomography (CT), which was introduced in the early 1970s, has also advanced to provide sufficient image quality for treatment planning. Since then, a new tech‑nique, known as “conformal radiation therapy”, in which a target is irradiated by conformal beams Figure 2　A medical linear accelerator (Linac) at the Juntendo University Hospital (Tokyo, Japan) (A) and multileaf collimator (B) placed at the aperture of the device, indicated by the white arrow.from several fixed directions, has emerged and is widely used. At the turn of the new millennium, a more sophisticated treatment technique, known as intensity-modulated radiation therapy (IMRT), has been introduced in this field6, 7), with develop‑ments in this technology advancing virtually every year. It can be used to treat patients using an acceptable dose distribution (Figure 3).Another advance in treatment is the introduc‑tion of image-guided radiation therapy (IGRT)8). IMRT has a steep fall-off of the radiation dose at the edge of the target volume. If the position of the target volume differs in a radiation session from the planning CT, the volume receives a lower dose than the plan prescribes. To overcome this problem, the position of the target is monitored before or during each treatment session using imaging modalities such as CT, magnetic resonance imaging (MRI), and/or ultrasound. Furthermore, it is possible to detect the movement of the target during irradiation. At Juntendo University Hospital in Hongo, tumor movement was tracked during a treatment session using the SyncTrax system