Calculation of energy-deposition distributions and microdosimetric estimation of the biological effect of a 9C beam

ArticleinBiophysik 48(2):135-43 · April 2009with7 Reads
Impact Factor: 1.53 · DOI: 10.1007/s00411-008-0206-8 · Source: PubMed

    Abstract

    Among the alternative beams being recently considered for external cancer radiotherapy, 9C has received some attention because it is expected that its biological effectiveness could be boosted by the β-delayed emission of two α particles and a proton that takes place at the ion-stopping site. Experiments have been performed to characterise this exotic beam physically and models have been developed to estimate quantitatively its biological effect. Here, the particle and heavy-ion transport code system (
    PHITS
    ) is used to calculate energy-deposition and linear energy transfer distributions for a 9C beam in water and the results are compared with published data. Although
    PHITS
    fails to reproduce some of the features of the distributions, it suggests that the decay of 9C contributes negligibly to the energy-deposition distributions, thus contradicting the previous interpretation of the measured data. We have also performed a microdosimetric calculation to estimate the biological effect of the decay, which was found to be negligible; previous microdosimetric Monte–Carlo calculations were found to be incorrect. An analytical argument, of geometrical nature, confirms this conclusion and gives a theoretical upper bound on the additional biological effectiveness of the decay. However, no explanation can be offered at present for the observed difference in the biological effectiveness between 9C and 12C; the reproducibility of this surprising result will be verified in coming experiments.