Photon Dose Calculations in a Fluence-Based Treatment Planning System: Data Processing, Implementation and Validation

Abstract: The accuracy of dose calculations for various aspects of a treatment planning system (TPS) using an energy fluence beam modelling has been evaluated. The investigated TPS uses a beam model, which separates the energy fluence into a number of sources in the treatment head. The energy fluence components are transported towards the patient and, together with the scatter generated in the patient, yield the total dose. The parameters in the model on which the calculations are based are derived from measured data. Several steps must be performed before a TPS can be adopted for clinical use. The most basic step is to collect the data required for the dose calculation algorithm of the TPS, such as depth doses, dose profiles, output factors in air and in water, etc. In this work, an investigation has been made of different methods of measuring output factors in air. An investigation on the variability of the prescribed characterisation measurements among several users has also been conducted. The treatment unit characterisation step involves taking the measured input data, stated by the TPS vendor, and converting these into the model parameters required by the TPS dose calculation model. TPS-calculated dose distributions for the standard characterisation fields have been evaluated and compared with the input dose measurements. This constitutes the basic quality control level for the individual beams. In clinical settings, a variety of accessories in terms of field shaping, compensators, wedges, etc, are commonly used. These different treatment scenarios have been evaluated in terms of the level of accuracy achievable. For a number of situations, e.g. beams in heterogeneous media, modulated and asymmetric beams and beams subject to different scattering volumes, the TPS dose calculations have been compared with measured and Monte Carlo simulated data. With the exception of dose calculations in the build up region and for some 60-degree wedge data, the dose calculations agree with measurements to within 3 % level.