Evaluation of a commercial radiation oncology treatment planning system against Monte Carlo simulated dose distributions

[摘要] English: A method is described in this study whereby dose distributions calculated by a treatment planning system (TPS) were evaluated by using dose distributions calculated with Monte Carlo (MC) simulations. The MC calculated dose data were used as a benchmark. A generic Siemens MD 2 linear accelerator was simulated with the BEAMnrc MC code to obtain beam specific dynamic variables in a phase space file (PSF) related to particle fluence in a plane at a known distance from a water phantom. Dose distributions from various field sizes were produced by simulations with the DOSXYZnrc MC code. Two datasets were produced consisting of percentage depth dose (PDD), profiles and diagonal profile data for 6 and 15MV x-ray beams. The CadPlan TPS was commissioned with these datasets for both energies. Analyses of TPS calculated dose distributions were done in a water phantom and dose distributions for various clinical cases on patient CT data. Patient CT datasets were transformed into patient CT models that were suitable for dose calculations with DOSXYZnrc. These models consisted of various media with various densities for which interaction cross section data is available. Dose distributions for a number of clinical treatment plans could be devised on both the TPS and DOSXYZnrc. These included head and neck, breast, lung, prostate, oesophagus and brain plans. Calculations on the TPS were done for the Single Pencil Beam (SPB) and in some cases the Double Pencil Beam (DPB) convolution algorithms in combination with the Batho and ETAR (Equivalent Tissue-air ratio) inhomogeneity correction algorithms. Dose distributions were normalized to the depth of maximum dose (dmax) for single fields and to the ICRU reference point in full treatment plans. The location of these points was the same for the TPS and DOSXYZnrc distributions. PDD curves, beam profiles, dose-volume histograms (DVHs) and equivalent uniform doses (EUDs) were produced to aid in the evaluation of the TPS dose calculation accuracy. Results demonstrated that the assumptions in the convolution models used to produce beam penumbra regions, especially in blocked field cases, fail to account for scattered dose contributions outside the treatment field and overestimated the dose underneath small or thin shielding blocks. The PB algorithms in combination with the inhomogeneity corrections show total disregard for lateral and longitudinal electron transport through heterogeneous media. This effect is pronounced in regions where electronic equilibrium is not found, like low density lung. This region, in combination with high density bone nearby, proved even larger discrepancies as dose absorption decreases in low density media and increases in high density media. A small 15 MV field passing through lung tissue exhibited large dose calculation errors by the PB algorithms. The dataset produced here is flexible enough to be used as a benchmark for any TPS utilizing commissioning measurements in water. This method can address commissioning results as well as any clinical situation requiring dose calculation verification.

[发布机构] University of the Free State