11. Comprehensive Study on the Fragment Reaction of Relativistic Heavy Charged Particles for Heavy Ion Radiotherapy

Naruhiro Matsufuji, Toshiyuki Kohno and Tatsuaki Kanai

Keywords: beam quality, fragments, LET, particle identification, radiotherapy

The production of projectile fragments is one of the important, but not yet completely solved, problems to be considered when planning for the utilization of high-energy heavy charged particles for radiotherapy. The aims of this study are to experimentally investigate fluence of the fragments and LET spectra produced from various incidents to elucidate the physical quality of the beams. The results are also compared with those by a fragment reaction simulation code to identify weakness of the code (L. Sihver et al.: Jpn. J. Med. Phys. 18, 1-21 (1998)).

An experiment was carried out at a beam port for biological experiments at HIMAC. Incident beams were as follows: 150 MeV/n ⁴He; 290 MeV/n ¹²C; 400 MeV/n ²⁰Ne; 490 MeV/n ²⁸Si; and 550 MeV/n ⁴⁰Ar. A beam was broadened in the same manner as in the case of therapy, i.e., both laterally and axially by a pair of wobbler magnets and a ridge filter, respectively.

PMMA, as a substitute for the human body, was used as a target. A binary filter made of PMMA plates was installed 300 mm upstream from the irradiating point. A E-E counter telescope with a NE102 plastic scintillator (5 mm in thickness) and a BGO scintillator (300 mm in thickness) was positioned at the irradiating point to identify the kind of fragment particles based on differences in the elements. To monitor any gain drifting of the E-E counters, stabilized green light emitted from a 1N6094 LED was used.

A gas-flow proportional counter was combined with the counter telescope system to measure LET spectra. P10 (Ar-90% / CH₄-10%) was used as a counting gas because it has the most uniform gas multiplication among other easily-obtained gases. Besides, stopping power of P10 gas relative to water for carbon ions can be regarded as being constant within 3 % even in a non-relativistic energy region. The thickness of the gas layer was 5 mm at NTP. Energy loss of a 290 MeV/n beam of carbon ions in the 5 mm thickness of P10 gas equals that in a 6.9 m thick film of liquid water, which is close to the thickness of an individual cell.

The energy of the primary particles after passing through any thickness of PMMA was deduced by comparing its depth-dose profile measured with a parallel-plate ionization chamber with results calculated by the fragment reaction simulation code.

Response of the BGO and NE102 scintillators were obtained in this energy region for the first time. Particle-species dependency of the responses was parameterized by AZ² of the incident particles. Fragment particles produced between incident particles and the PMMA target were well identified down to hydrogen by the E-E scatter plot, and the fluence of each fragment element was deduced. The difference between experimental and simulation code results suggests the need for theoretical research and the establishment of a reliable nuclear reaction model in this energy region. The LET spectra were also derived for each element (Fig.9). These results indicate that the greater part of the dose is delivered by primary particles, though many light fragments are produced.

Fig.9. LET spectra of 400 MeV/n - ²⁰Ne beam fir each fragment element at PMMA thickness of 90mmw-eq.

Publications:
1)Matsufuji, N., Kanai, T. and Kohno, T. submitted to Rad. Res.
2) Matsufuji, N., Kanai, T., Komami, H. and Kohno, T. Nucl. Instr. and Meth. In Phys. Res. A in press.
3) Matsufuji, N., Kanai, T., Komami, H. and Kohno, T. Nucl. Instr. and Meth. In Phys. Res. A 430, 60-68, 1999.
4) Kohno, T., Kiyota, T., Matsufuji, N. and Kanai, T. Nucl. Instr. and Meth. In Phys. Res. A in press.