Studies of clinical proton dosimetry using Monte Carlo simulation and experimental techniques

University dissertation from Stockholm : Medical Radiation Physics, Stockholm University, together with KI

Abstract: The present work aims at improving clinical proton dosimetry based on ionization chambers, and to bring its state of development closer to the corresponding status of conventional photon and electron dosimetry. In order to accomplish this, both theoretical and experimental methods have been used.A Monte Carlo code (PETRA) was developed in order to calculate stopping-power ratios water/air for proton beams of therapeutic interest (50-250 MeV). Special attention was paid to the contribution from secondary electrons generated along the proton track and secondary protons emitted in non-elastic nuclear interactions. When the contribution from secondary electrons was included, differences of about 0.5% were found compared to the result for protons only. The inelastic nuclear interactions were not found to affect the calculated stopping-power ratios water/air, and no significant change was obtained when the stopping-power ratio was calculated from the complete proton fluence distribution compared to using the concept of effective energy. The nuclear interactions have, however, a large influence on the proton depth-dose distribution due to a removal of protons from the beam. The peak-to-plateau ratio was found to decrease by up to 35% for a 250 MeV incident monoenergetic proton beam compared with a calculation without nuclear interactions. The latter result shows the importance of a correct inclusion of inelastic nuclear interactions in any Monte Carlo simulation intended for clinical applications.In the experimental part, different calibration methods used within ionization chamber based dosimetry were evaluated, including an investigation of different types of ionization chambers. Calibration in terms of absorbed dose to water was for the first time adapted to proton dosimetry, and the consistency in the determined absorbed dose compared to other calibration methods encourages the implementation of this formalism in clinical proton dosimetry. When measuring the absorbed dose in a proton beam with two IC-18 ion chambers, the values obtained were up to 1.5% lower compared to the values obtained using other well-investigated ionization chambers when the IC-18 chambers were calibrated in terms of air kerma free in air. This discrepancy disappeared when the chambers were calibrated relative to a reference chamber (NE-2571) in a high-energy electron beam instead, a result which calls into question the use of the kattkm-value given in the IAEA Code of Practice for the IC-18 chamber. In an additional work, an improved implementation of Faraday cups within proton dosimetry was studied. This new method avoids the problem of low-energy collimator-scattered protons that otherwise increase the uncertainty due to an increase in the energy spread of the beam.

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