Cellular dosimetry and microdosimetry for internal electron emitters

radio-biological descriptions of cellular dosimetry and microdosimetry require both radiation dose and radiation quality. The lineal energy, defined as a ratio of the energy deposition by a particle in the biological target and the mean chord length of this target, is generally adopted to characterise the radiation quality. Most microdosimetry applications assume that the cell nucleus is the target region. Therefore, the lineal energy is obtained for the source (S) to target (T) geometry, T S, where S5 cell surface, cytoplasm, cell nucleus and T5 cell nucleus. The definition of lineal energy is based on the approximation that the particle mean path length is equal to target mean chord length. This approximation is valid for crossers of external irradiations. In the case of starters, insiders and stoppers of internal sources, particle path lengths are always shorter than target chord lengths. Thus, the lineal energy does not reflect the specific energy deposition along particle path. In the present work, the specific energy deposition in a target is calculated using three distance parameters, i.e. target mean chord length, particle mean path length in the target and particle individual path length in the target. Monte Carlo calculations are performed for electrons of various energies and cells of different sizes. Results are analysed and discussed. (authors)