Using Monte Carlo techniques to evaluate the dose distributions from a radiotherapy treatment planning system
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Date
2000-06
Authors
Awusi, Kavuma
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
English: In this study we used Monte Carlo techniques to simulate the SL25 linear accelerator
treatment head using the BEAM Code. The main purpose of study was to evaluate the
dose distributions obtained by the CADPLAN treatment planning system (TPS) for 8 MV
photon beams of a SL25 linear accelerator in realistic patient models. Simulation of the
treatment head involves modeling of the main components of the treatment head that
have influence on the absorption and scattering of radiation. Simulation of the accelerator
was done in two parts to minimize the simulation time.
Analysis of the data generated by the BEAM code was carried out using BEAMDP,
another subsidiary of the BEAM code. We calculated the beam characteristics which are
difficult to measure experimentally, such as angular distributions, spectral distributions,
planar fluence and planar energy fluence at a plane located just above the jaws of the
treatment head.
The phase space files at the isocenter were used as source input for DOSXYZ, a MC code
to calculate 3D dose distributions in water or CT based phantoms. The DOSXYZ code
was used to calculate depth dose and cross plane profiles in a water phantom. The data
obtained with Monte Carlo methods were compared with that obtained by ionization
chamber measurements. Depth dose and cross plane profiles obtained by Monte Carlo
methods and ionization chamber measurements generally agreed within 2%.
We created patient models from CT data of real patients using the CTCREATE option of
the DOSXYZ program. Dose distributions for a number of field sizes and different
anatomical sites were calculated with the DOSXYZ code and compared with
corresponding dose distributions calculated by the TPS. The modified BATHO and
ETAR inhomogeneity correction methods used in the TPS were evaluated.
Results show that Monte Carlo methods can accurately reproduce ion chamber
measurements in a water phantom. Monte Carlo techniques are very useful for evaluating
the accuracy of dose distributions generated by treatment planning systems in patient
based models where measurements are impossible. The BATHO and ETAR methods
showed comparable results to the Monte Carlo results. This could be due to the
inefficiency of the method (visualization of the dose distributions) that we used for the
comparison of the results. A more quantitative method like the use of the dose difference
volume histogram could give a more comprehensive evaluation.
Description
Keywords
BEAM code, DOSXYZ code, Accelerator simulation, Photon beam and dose distributions, Monte Carlo method, Radiation dosimetry, Radiation -- Measurement, Dissertation (M.Med.Sc. Medical Physics))--University of the Free State, 2000