7. Studies on Dosimetry for Therapeutic Carbon Beams

Akifumi Fukumura, Takeshi Hiraoka, Kaname Omata, Mitsue Takeshita, Yutaka Noda, Kiyomitsu Kawachi, Tatsuaki Kanai, Takeshi Murakami, Naruhiro Matsufuji, Yasuyuki Futami and Guenther H. Hartmann^*
(^*DKFZ, Germany)

Keywords: carbon beam, dosimetry, range, fragmentation

Since 1994, the National Institute of Radiological Sciences (NIRS, Japan) has been carrying out clinical trials of cancer treatment using high-energy carbon ion beams. Establishing carbon beam dosimetry is essential to performing the trials. Objects of this studies include absolute dosimetry, relative dosimetry, range measurements and beam attenuation due to nuclear fragmentation.

Regarding the absolute dosimetry, we employed three different methods, ionization chamber, calorimeter and fluence measurement methods. Measurement were performed at zero depth for 290 MeV/u carbon beams with the ionization chamber and fluence methods, and at a depth of 12 cm in water for 6-cm modulated 290 MeV/u carbon beams with the calorimeter and ionization chamber methods. Values obtained with the different methods were in good agreement with each other. In particular, the discrepancy between results by the ionization chamber and calorimeter methods was less than 1 %. We also carried out dosimetry intercomparison between two groups from different carbon beam facilities, NIRS and GSI-DKFZ in Germany. Values estimated individually by each group were in good agreement, within 0.5 %. The consistency established an international common framework for carbon beam dosimetry.

Carbon beam treatment requires not only absolute dosimeter, but also relative dosimetry. Several characteristics of small p-type silicon diodes were investigated with irradiation of heavy ion beams. The diodes showed favorable characteristics as relative dosimeter in terms of stability for carbon beam irradiation. The diodes were applied to practical verification of the treatment planning in the pre-clinical examination carried out at HIMAC.

In an ordinary range measurement, a combination of a phantom and detector is employed. Such a measurement requires much time and an assumption of constant range during it. We developed a simple range measurement method using visible rays generated in a bare plastic scintillator block. This method allows real-time measurements with high spatial resolution of less than 0.5 mm.

We also investigated nuclear fragmentation of carbon beams. Attenuation of carbon beams caused by fragmentation in an energy absorber may possibly change the fluence of carbon beams which is planned for patient irradiation. The survival of primary carbon beams after passing through an absorber was measured with E plastic scintillator. The results showed that polyethylene and PMMA were water-equivalent in terms of the nuclear reactions and appropriate as energy absorbers to shift the range of the primary beams. The charge-changing cross sections of several materials for carbon beams were also deduced from the slope of the attenuation.

Publications:
1) Fukumura A.: Doctoral dissertation, Tohoku Univ., 1999
2) Fukumura A., Hiraoka T., Omata K., Takeshita M., Kawachi K., Kanai T., Matsufuji N., Tomura H., Futami Y., Kaizuka Y. and Hartmann G. H.: Phys. Med. Biol. 43, 3459-3463, 1998.
3) Fukumura A., Noda Y., Omata K., Hiraoka T., Futami Y., Matsufuji N., Murakami T. and Kanai T.: Nucl. Instr. and Meth. in Phys. Res. A 416, 148-151, 1998.