Monte Carlo Calculations of Energy Deposition on Cellular, Multicellular and Organ Level for Auger Emitters

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The radiation dose to patients from injected radiopharmaceuticals is usually calculated as an average dose to target organs. Uniform distribution of radiopharmaceuticals is presumed and the highly localised character of energy deposition from Auger emitters is ignored. In reality many common diagnostic agents are not distributed uniformly and also the contribution made by intracellular Auger electrons to the organ radiation dose should be characterised on the microdosimetry scale. The goal of this paper is to compare Monte Carlo (MC) calculations of energy deposition with point-computational techniques at the cellular, multicellular and organ levels. Point-computational techniques are inherently faster, they are restricted to homogenous tissue regions, mathematically described by simple analytical geometry. MC techniques are slower; they provide, however, not only accurate estimates of local energy deposition, but demonstrate also the stochastic variability in small target volumes. They are also able to treat heterogeneous tissue regions containing complex geometric shapes like the DNA molecule. The following conclusions can be drawn from the calculation performed and their comparison with other results: (1) At the cellular level there is no need to take into account the stochastic variation of absorbed dose delivery, the results of 'classical' and MC approaches agree quite well. (2) MC results for self-dose and cross-dose delivered in the case of whole organ analysis also agree well with the results obtained using the scaled electron dose point kernels tabulated on the assumption of an isotopic point source.