Dose measurements in therapeutic proton beams

The number of cancerous diseases is increasing every year and about two thirds of all cancer patients need to be treated with radiation therapy. In curative as well as palliative treatments, the objective of the radiation therapy is to deposit the dose of the radiation with maximum accuracy to the volume of the tumor in order to destroy cells of the cancerous tumors or to prevent further cell division aiming at the same time at protecting the healthy epithelium and avoiding future consequential damages from the radiation. Within the scope of this thesis measurements of the radiation dose have been performed with two therapeutic proton beams (OPTIS and proton gantry) at the Paul Scherrer Institute in Villigen (Switzerland). The objective was to investigate the dose distribution from the therapeutic proton beam within the beam and to determine its biological effectiveness targeted on a specified volume in order to achieve an optimum therapeutic treatment. Because of their depth dose distribution, protons and ions are an excellent tool for radiation therapy. Contrary to photons, which show an exponential decrease in a narrow area at the end of their coverage, protons and ions deposit almost the total of their energy, at the so-called Bragg Peak. The location of the Bragg Peak depends on the primary energy of the particles. A monoenergetic proton beam cannot be used for therapy, therefore the beam needs to be adjusted with the help of different components of the beam line system, like modulators, between the accelerator and the patient in such a way that the targeted volume will receive optimum radiation and the healthy epithelium will be protected to the maximum possible extent. The determination of the biological effective dose (absorbed dose x relative biological effectiveness (RBE)) is of utmost importance for the ion therapy as the relative biological effectiveness is an integral part of the planning for radiation therapy. At prevent this is not the case in proton therapy. A constant dose of energy is deposited at the volume of the tumor. However, the biological effectiveness of the radiation not only depends on the absorbed dose but also on the linear energy transfer (LET) of the radiation. Therapy with protons takes for granted a constant absorbed dose and a constant LET in the target volume. However, experiments with cells and measurements with thermoluminescence dosemeters (TLDs) using the HTR (high temperature ratio)-method, developed at the Atominstitut of the Austrian Universities, have shown that the 'average LET' and thus the 'biological relevant dose' (absorbed dose x HTR) in the range of constant absorbed dose (Spread Out Bragg Peak) increases in the modulated beam. The thermoluminescence dosemeters were calibrated individually to be in a position to indicate the absolute absorbed dose. In total four series of measurements in a tissue equivalent phantom made from polyethylene were performed in the therapeutic proton beams at the Paul Scherrer Institute. The depth dose distribution, the 'biological relevant dose', the radial distribution of the dose and the 'average LET' were determined. The results were compared either with results from the ionization chambers of PSI or with results from literature. The depth dose curves, determined with the thermoluminescence dosemeters and the ionization chambers as well as with simulation calculations showed a good conformity. Measurements of the 'biological relevant dose' showed a significant increase of biological effectiveness close to the end of the Spread Out Bragg Peak. The 'average LET' was determined by means of the HTR-values with thermoluminescence dosemeters and the tissue equivalent proportional counter PART-II (LET-spectrometer) as well as by simulation calculations and the results were compared with data from the literature. The 'average LET', determined by measurements with the thermoluminescence dosemeters, resulted in a value of 7 to 8 keV/μm and the value determined with the LET-spectrometer was approx. 5,3 keV/μm. The radial distribution of the dose within the OPTIS-beam showed a non-homogenous profile of the dose. Possible reasons for this will be discussed in the thesis. The existence of the thermal neutrons has been proved at the OPTIS-plant as well at the proton gantry. Their contribution to the total dose of energy at the OPTIS-plant is approx. 0,15 mGy and at the gantry approx. 0,35 mGy. On basis of measurements at the gantry it is assumed that fast neutrons will contribute 90 mGy to the absorbed dose in the Spread Out Bragg Peak. Therefore, thermoluminescence dosemeters constitute a helpful and reliable instrument for measurements of the dose in radiation therapy because information from the measurements can be used for a variety of purposes. At a correlation between values of the relative biological effectiveness and the HTR-values it could be possible in future to derive a measurement for biological effects by means of physical detectors. (author)