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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-03192009-104915 |
| Date | 19 March 2009 |
| Creators | Shaw, William |
| Contributors | Dr FCP du Plessis |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-03192009-104915/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
Page generated in 0.0052 seconds