| Abstrakt: | In this work models of two therapeutic beams of high-energy X-rays and protons
were elaborated. The purpose of the investigation was the determination of significant
parameters used in clinical dosimetry, basing on the elaborated models. The calculations
were performed by means of computer simulations on the ground of the Monte Carlo
algorithm.
At the first stage of this work the head model of the Clinac 2300 linear accelerator
by Varian was elaborated with the use of the professional MCNPX code. Verification of
this model was carried out using the comparison between the calculated and measured
depth-dose distributions in the standard dosimetry medium – water. The depth-dose
distributions in water were determined for SSD = 100 cm and for three square radiation
fields of 3 cm x 3 cm, 10 cm x 10 cm and 40 cm x 40 cm. Successful validation of the
prepared model with the measuring system model made it possible to begin the next part
of this investigation in which the energy spectra along the central axis of the beam and in
the direction perpendicular to this axis in water were calculated. The spectral calculations
were performed for the therapeutic 20 MV X-ray beam. The influence of the radiation
field sizes on the shapes of the spectra in water were determined and the mean energy of
the beam was calculated for the chosen irradiation conditions.
At the second stage of this work the proton beams and the PTW 23343 Markus
type ionization chamber used in clinical dosimetry were modeled. On the base of these
models the perturbation factors 𝑝𝑤𝑎𝑙𝑙 , 𝑝𝑐𝑎𝑣 and the total perturbation factor 𝑝𝑞 were
calculated for the punctual proton beam with energies of 15 MeV, 30 MeV, 60 MeV, 80
MeV. The influence of energy and spatial spread on the perturbation factors was
determined. Moreover, the model of the real beam based on the passive system forming
the proton beam in the Institute of Nuclear Physics of Polish Academy of Sciences in
Bronowice was elaborated. The investigation basing on this model indicated that the
depth-dose distributions obtained with the use of the water logic detectors and with the
Marcus chamber model are compatible and they agree with the experimental results. |