Since the high energy accelerator plan was changed from a 40 GeV direct machine to a 12 GeV cascade one, a 500 MeV rapid cycling booster synchrotron was installed between the injector linac and the 12 GeV main ring at KEK, National Lab. for High Energy Physics. The booster beams were used not only for injection to the main ring but also for medical use. Their energy was reduced to 250 MeV by a graphite block for clinical trial of cancer therapy. In 1970's,π^- or heavy ions were supposed to be promising. Although advantage of protons with Bragg Peak was pointed out earlier, they seemed effective only for eye melanoma at that time. In early 1980’s, it was shown that they were effective for deep-seated tumor by Tsukuba University with KEK beams. The first dedicated facility was built at Loma Linda University Medical Center. Its synchrotron was made by Fermi National Accelerator Lab. Since a non-resonant accelerating rf cavity was installed, operation of the synchrotron became much easier. Later, innovation of the cyclotron was achieved. Its weight was reduced from 1,000 ton to 200 ton. Some of the cyclotrons are equipped with superconducting coils.

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We have developed an Electron-Tracking Compton Camera (ETCC) for medical imaging due to its wide energy dynamic range (200–1,500 keV) and wide field of view (FOV, 3 str). This camera has a potential of developing the new reagents. We have carried out several imaging reagent studies as examples; (1) ¹⁸F-FDG and ¹³¹I-MIBG simultaneous imaging for double clinical tracer imaging, (2) imaging of some minerals (Mn-54, Zn-65, Fe-59) in mouse and plants.
In addition, ETCC has a potential of real-time monitoring of the Bragg peak location by imaging prompt gamma rays for the beam therapy. We carried out the water phantom experiment using 140 MeV proton beam, and obtained the images of both 511 keV and high energy gamma rays (800–2,000 keV). Here better correlation of the latter image to the Bragg peak has been observed. Another potential of ETCC is to reconstruct the 3D image using only one-head camera without rotations of both the target and camera. Good 3D images of the thyroid grant phantom and the mouse with tumor were observed.
In order to advance those features to the practical use, we are improving the all components and then construct the multi-head ETCC system.

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Cone-like acryl converters have been used for transforming the energy-distribution information of incident fast neutrons into the spatial-distribution information of recoil protons. The characteristics of neutron-proton conversion have been studied up to around 10 MeV by using an imaging plate (IP). A notable and interesting signal enhancement due to recoil protons generated in an acrylic converter was observed on IP images for irradiation with a ²⁵²Cf source. A Monte Carlo calculation was carried out in order to understand the spatial distributions of the signal enhancement by recoil protons; these distributions promisingly involve the energy information of incident neutrons in principle. Consequently, it has been revealed that the neutron energy evaluation is surely possible by analyzing the spatial distributions of signal enhancement that is caused by recoil protons.

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Recent advance in the field of radiation oncology, especially in medical physics for radiation therapy (RT), has considerably improved treatment outcomes of various cancers including prostate cancer with regard to both of tumor control and morbidity. Three-dimensionally conformal RT with image-guided radiotherapeutic modalities for accurate tumor localization, such as brachytherapy, intensity-modulated radiation therapy (IMRT), and charged particle beam RT can thereby deliver a large dose to the tumor and allow the sparing of surrounding normal tissues.
It is thought that prostate cancer is one of representative cancers which have been treated with RT as a curative intent and benefited from novel conformal RT techniques. Because the number of prostate cancer patients has been increasing year by year in Japan as results from wide spread of PSA screening and rapid change in life style, RT has been recently playing much more important roles in the curative treatment for patients with prostate cancer.
Hence, we will review the outcomes of RT for prostate cancer and introduce the benefit of modern RT modalities from clinical aspect. In addition, our future prospect to further yield better disease control with minimum morbidity compared with present RT will be also mentioned in the report.

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Positron emission tomography (PET) plays important roles in cancer diagnosis and molecular imaging research; but potential points remain for which big improvements could be made, including resolution, sensitivity and costs. For example, the sensitivity of present PET scanners does not exceed 10%. This means that more than 90% of the gamma-rays emitted from a subject are not utilized for imaging. Therefore, research on next generation PET technologies remains a hot topic worldwide. In this paper, we introduce some research trends by describing PET physics research in the National Institute of Radiological Sciences (NIRS). A depth-of-interaction (DOI) detector, for which various methods have been studied, will be a key device to get any significant improvement in sensitivity while maintaining high spatial resolution. DOI measurement also has a potential to expand PET application fields because it allows for more flexible detector arrangement. As an example, we are developing the world's first, open-type PET geometry “OpenPET”, which is expected to lead to PET imaging during treatment. The DOI detector itself continues to evolve with the help of recently developed semiconductor photodetectors, often referred to as silicon photomultipliers (SiPMs). We are developing a SiPM-based DOI detector to achieve sub-mm spatial resolution, which is reaching the theoretical limitation of PET imaging.

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