Dose to tissue medium or water cavities as surrogate for the dose to cell nuclei at brachytherapy photon energies

Assuming that the cell nucleus with its DNA content is the most important target for biological response, the aim of this study was to investigate, by means of Monte Carlo (MC) simulations, the relationship of the dose to a cell nucleus in a medium, MC-D_n,m, to dose to the local medium, MC-D_m,m, and dose to a water cavity embedded in the medium, MC-D_w,m. This was carried out for different combinations of cell nucleus compositions and tissue media, for different photon energies used in brachytherapy and compared to cavity theory calculated D_w,m. Our results showed that for the nucleus compositions and tissue types investigated, MC-D_w,m is a good substitute to MC-D_n,m. Whether other studied surrogates are good approximations to MC-D_n,m depends on the target size, target composition, composition of the surrounding tissue and photon energy. Elemental compositions of the tissue and nuclei affect considerably the absorbed dose to the cell nuclei for low energy brachytherapy sources.

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