Electron Beam Adjustment in PLATO RTS 2 Including the Effect of Air Gaps

Background and Purpose: Beam characterization for electron dose calculations in PLATO RTS 2 treatment planning system requires the tuning of two adjustment parameters: sqx (the initial angular spread) and FMCS (a "fudge" multiple Coulomb scattering parameter). This work provides a set of suggestions to optimise electron dose calculations with PLATO, taking into account the effect of air gaps between the electron applicator and the patient skin. Material and Methods: Two adjustment criteria have been followed: one which uses just one input data set corresponding to the standard (null) air gap and another one that takes into account the whole range of clinically used distances between the electron applicator and the patient surface. The adjusted values of sqx were compared with experimental data and GEANT3 Monte Carlo code results. A systematic study has been carried out of the effect of both adjustment parameters on electron dose calculations in water. Comparisons of dose distributions and point dose values have been done between PLATO RTS2, GEANT3 Monte Carlo code and experimental data. Also the dependence on field size has been assessed. The values of sqx for the different electron energies obtained through the different approaches are discussed. Results and conclusions: The first adjustment criteria yield unrealistic dose distributions whenever the air gap is different from the standard one. A sqx balanced with a proper FMCS parameter leads to reasonably good dose distributions and point dose values that agree with experimental results within less than 1%.