Two types of passive radiation dosimeters of the radiochromic film and the alanine have been extensively and regularly applying for dose measurements in photon fields of medical X-rays and gamma-rays. This dissertation studies these two passive dosimeters for dose measurement in therapeutic beams of boron neutron capture therapy (BNCT), proton therapy, and carbon ion therapy. Responses of these two dosimeters irradiated with various particles differ from their photon responses. Therefore, this dissertation mainly investigates the relative effectiveness (RE) values of these two dosimeters for various particles in relation to photons, so as to make these dosimeters be used for dose measurement in the context of hadron therapy.
In the epithermal neutron beam used for BNCT, the neutron dose to a dosimeter is induced by the charged particle emission of neutron reactions, and the gamma-ray dose to a dosimeter is also caused by electrons that are produced from photon interactions in a matter. The BNCT beam is therefore a complex mixed field. This dissertation places the radiochromic film and the alanine in a PMMA phantom irradiated with the BNCT beam at the Tsing Hua Open-pool Reactor (THOR) for dose measurement, and uses a Monte Carlo code of MCNPX to perform calculations. In order to make the calculated results an auxiliary, those calculations adapted a calibrated neutron monitor and the general source definitions of neutrons and gamma-rays to determine the neutron REs of both dosimeters for the BNCT beam.
Since the radiochromic film is a two-dimensional (2D) dosimeter, this dissertation places a large piece of film in the PMMA phantom to measure the 2D distributions of the gamma-ray dose component of the BNCT beam. Regarding the determination of the neutron dose to the film, it can be divided into three steps: first was to measure the disintegrated radiations of the activated copper using the combination of the indirect neutron radiography (INR) and the image plate (IP), to retrieve the 2D distributions of the radioactivity over the activated copper, then use both MCNPX and the readings of the neutron monitor to calculate the neutron doses to the films on the central axis; lastly, the results of INR plate measurements were applied to expand the neutron doses to the film over the film’s area. Subtract the 2D distributions of the neutron doses from those of the photon equivalent doses to have the 2D distributions of the gamma-ray doses to the films. The measured results were generally in a good agreement with the calculated results of MCNPX. Meanwhile, if the composition of the neutron and gamma-ray sources is constant in the BNCT beam, the fractions of neutron and gamma-ray fluxes would not vary too much. Thus the distributions of the photon equivalent doses measured using the radiochromic films can be directly applied to the quality assurance (QA) procedure of the beam uniformity of the beam line. This dissertation also attempts to set up the criterion for the QA measurement.
Of the measurements in therapeutic beams of proton therapy and carbon ion therapy, this dissertation mainly discusses the RE values of both dosimeters located at the measured positions. First was to review and conclude the RE functions of energy of both dosimeters for protons, carbon ions, and other particles. In which the RE functions of energy of the film for particles were originated from related experiments, while those of the alanine were modeled and calculated using the track structure theory. Because the REs of these two dosimeters for particles depend on particle energy; hence, in order to determine the RE of the dosimeter at a specific measured position, the energy distribution of a certain particle must be known. This dissertation uses MCNPX to calculate the energy distributions of particle fluxes at different positions along the Bragg curves. Then the RE of the dosimeter placed at the certain position can be determined by using the dose weighted method with the energy function of RE. Owing to that the external ion beam therapy utilizes the doses in the region of the spread-out Bragg peak (SOBP) to treat, this dissertation therefore uses MCNPX to calculate Bragg curves of multiple energies in liquid water, and generates several SOBPs by sets of weighting factors that were referred to a literature. The RE of dosimeters at each position in the SOBP region was determined using the RE distribution along each Bragg curve. Results showed that the proton REs of both the radiochromic film and the alanine dosimeter are fixed values, which are around 0.96 for the film and almost unity for the alanine. As for the measurement in therapeutic carbon ion beams, not only the carbon ions but the sufficient nuclear fragments are present in the vicinity of the Bragg peaks. The magnitude of fragments depends on the incident carbon ion energy and measured position, thus it has to acquire the RE functions of energy for multiple particles to correct the REs of both dosimeters for specific position and incident energy before these two dosimeters can be used to measure the delivered dose of carbon ion therapy. However, it is so complicated that the dose measurement using these dosimeters will not be viable.
Accordingly, this dissertation has studied the RE values and practical usages of the aforesaid passive dosimeters applied to three therapeutic beams of hadron therapy.