Microdosimetry calculations for low-LET bystander experiments with microbeams

Experimental results on bystander effects induced by low-LET radiation are becoming available through application of focused X-ray sources and electron microbeams. Microdosimetric quantities useful in experiments with these facilities are obtained from track-structure simulations; our enhanced PITS code simulates, event-by-event in liquid water, interactions of mono-energetic electrons or fluorescent X-rays at primary energies up to 100 keV. Soft X-ray capability is extended to photon focusing and Bremsstrahlung production. Energy deposition is scored in virtual spheres at varied lateral (r) and forward (h) penetrations, for sphere diameters appropriate to the energies of the secondary electrons. Mean energy deposited at (r,h) is calculated from the distributions in event size. Characterization of energy deposition by low-LET radiation in terms of the spatial dependence of event frequency and mean event size in micron-sized targets enables rapid calculation of the mean dose to particular cells in complex geometrical arrangements. The cells of interest may be part of a multi-cellular target, e.g., an explant or acinus, where the difference between dose to bystanders and to cells targeted by the microbeam is an important aspect of the experimental design. Beam energy and size, window thickness, and depth into the sample have a large impact on the amount of energy delivered to particular cells. This mean-value approach will be useful when the volumes of interest are large enough to diminish the importance of statistical fluctuations or the number of variables makes the Monte Carlo method impractical. We are developing interactive computational tools, including a graphical user interface, that incorporate the event frequency, mean specific energy, and density of energy deposited in complex cellular targets. The computations are controlled by user?s choice of number and energy of primary particles, and target shape and size. The total energy deposited in cGy in the volume of interest is digitally displayed