Abstract
A dosecalculation formalism is presented that is based on first principles and facilitates techniques for modelling tissue and shielding effects. The calibration is based on Monte Carlo simulations of the ratioof dose to water overkerma to air for the source-applicator geometry. A given source can then be calibrated by measuring the air kerma rate for the calibration point. The Monte Carlo simulation enables the primary and scatter dose fractions to be scored separately. The distributions are then parameterized using physically based functions. As electron ranges in the brachytherapy domain are negligible, the primary dose in treatment planning can be calculated directly from the parameterization using the radiological path as length variable. The scattered photons have, however, a considerable mean free path and their dose deposition response to heterogeneities is more complicated. Neglecting the change in scatter dose, e.g.dueto a superficial location of thesource, may cause severe overestimation of the total calculated dose. The scatter dose deposition can be modelled by means of scatter kernels that represent the energy deposition around a primary photon interaction site. The useof parameterized primary and scatter dose distributions is directly applicable in treatment planning systems while the use of scatter kernels requires further development before clinical implementation.
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