Search

Carbon ion radiotherapy has been conducted since 1994 at National Institute of Radiological Sciences, Japan, with beams from Heavy Ion Medical Accelerator in Chiba. Accumulated number of patients treated will become more than 5,000 by the end of this year. The tumor sites with more than 200 treated patients include Prostate, Lung, Head and Neck, Bone and Soft Tissue, and Liver. The experience of 15 years can be summarized as that carbon ion therapy has established efficacy in treatment of tumors, especially so-called ‘radio-resistant’ ones and including ‘inoperable’ cases. Reduction of fractionation, and thus treatment period, is another important achievement which can benefit patients in many ways. Recent development of the therapeutic system in order to advance more efficient and patient-friendly treatments will be presented also.

Results are reported on precise measurements of the weak-decay processes of Λ 4 H and Λ 4 He hypernuclei formed via the 4 He (K stopped − , π − ) reaction. The lifetimes of Λ 4 H (194 −26 +24 ps) and Λ 4 He (256 ± 27 ps) were obtained. The π 0 and π − decay branching ratios of Λ 4 He (0.515 ± 0.035 and 0.322 ± 0.034, respectively) were precisely determined and they are compared with recent theoretical estimates and related experimental values. Our results favor a Λ-nucleus interaction with an inner repulsive core. The non-mesonic weak decay widths of Λ 4 H and Λ 4 He are reexamined on the basis of our measurements and updated knowledge of the mesonic decay processes. It is found that the non-mesonic decay of Λ 4 He predominantly takes place via the Λ + p → n + p process.

Researchers explore the use of Bragg Ionization Peak in radiotherapy for tumors. Considerations for the production of a high quality radioactive beam include secondary beam production efficiency, primary beam survival, and multiple scattering. A Positron Emitting Beam Analyzing camera was developed and tested at BEVALAC using a neon-19 beam stopping point. Results indicate that in homogeneous brain tissues, CT measurements and radioactive beam measurements are between 1-2 mm; there can be substantial difference between CT measurements and radioactive beam estimates if the beam stopping point crosses the fossae; random coincidences due to detector activation also appear as diffused images in the image plane of the beam stopping point; and further enhancements are necessary to make reliable measurements in the human trunk and complex brain regions.

The NIRS heavy-ion two-synchrotron medical facility, HIMAC, was approved in the 1987 fiscal year and clinical trials were started in late June 1994 as previously arranged. The operation experiences show that the entire HIMAC facility can work well with high stability and excellent reproducibility. For example, the intensity of beam extracted slowly from the rings can be reduced as low as 500 particles per pulse because stable and reproducible acceleration can be achieved in the rings in spite of no beam feedback. This performance enables direct counting of the beam and its fragments in the preparatory experiments toward clinical treatments and provides a promising basis for acceleration and storage of radioactive beams toward simultaneous treatment and diagnosis in future.

Results are reported on a lifetime measurement of Λ4H formed in 4He (stopped K−,π°) reaction. Decay of Λ4H was clearly identified by observing the monochromatic π− peak from the two-body decay ( Λ 4 H → 4 He + π − p x =132.9 MeV/c ). The lifetime of Λ4H was determined to be τ(Λ4H) = 194−26+24 ps from a distribution of time difference between the K− injection and π− emission.

A heavy ion synchrotron complex for medical use is being constructed at Chiba, Japan. General feature and present status of this project are described.