| الوصف: |
Particle beam therapy, which uses accelerated protons and carbon ions to treat cancer, has become more widely available during recent decades. Due to the increasing dose deposition at the end of the particle���s range (Bragg Peak) and the sharp fall-off after it, compared to the conventional photon and electron beam radiotherapy, particle therapy has an advantage of treating tumors located close to the organs at risk (OAR) while sparing efficiently the surrounding healthy tissue.Using small animal models is essential to improve understanding of radiation effects in tissues. At MedAustron (Wiener Neustadt, Austria), it is planned in the near future to irradiate small animals in the frame of pre-clinical radiobiological research. The equipment (software and hardware) designed for patients has certain limitations and has to be adapted for small targets relevant to small animals. Accordingly, the main focus of this work was given to the irradiation of small fields with proton beams and to the investigation of using the existing patient-developed equipment for small field irradiation to apply it for in vivo animal studies. This work is part of the preparatory work intended to establish the whole irradiation workflow for radiobiological in vivo studies at MedAustron.This thesis is based on a simple phantom, made of polymethylmethacrylate (PMMA) material, representing in its size a small animal (e.g., mouse). The small targets inside the phantom were delineated, the doses were calculated and the treatment plans were created in the clinical treatment planning system (TPS-Raystation, Raysearch, Sweden). Experiments were performed in different setups. Since the beams are rather large in comparison to the target size, apertures are required to limit the irradiation field. Therefore, the apertures as beam shaping devices, with the openings of 1.5 cm and 1.2 cm in diameter, were tested. Moreover, the typical targets in small animals are located at shallow depths and a pre-absorber has to be included in the beam pathway to reduce the penetration depth of clinical beams. Two different types of pre-absorbers, i.e. either a bolus or a range shifter, were employed to find the optimal setup for the small field irradiation. In addition to different apertures and different pre-absorbers investigation, small targets with 1.2 cm and 0.8 cm radius were analyzed. The positioning accuracy of the aperture was also tested. Finally, an importance of including the aperture into the treatment planning process was addressed. During the irradiation, the absorbed doses were measured with two different detector types: a microDiamond detector (active detector) and EBT3 films (passive detector).The investigation in the setups with apertures of different sizes yielded that the dose distribution in the target was homogeneous and the lateral penumbra was significantly reduced. Based on the experimental approach with detectors, the dose was distributed homogeneously inside the target when the range shifter was used as the pre-absorber. When the bolus was placed in front of the phantom as pre-absorber, more dose was concentrated at the center of the target and the lateral dose profile became dome-shaped with a 10% higher dose than the prescribed dose. The target dose inhomogeneities may increase the risk of overdosing and underdosing different areas within the target volume. Regarding the target sizes, comparable dose distributions and lateral penumbras were present in both investigated targets of 0.8 cm and 1.2 cm in diameter. The positioning accuracy of the aperture relative to the phantom and the target was important and misplacements of more than 1mm led to fairly large differences in target doses. Last but not least, not considering the aperture in the TPS led to target overdose of 12%. In addition, when the aperture was not considered in the treatment planning process and simply placed in front of the phantom, three times larger lateral penumbra was observed. To conclude, the clinical beamline has to be adapted for small (< 2cm) and shallow field irradiation relevant in small animals using a collimator and a pre-absorber. The use of clinical TPS based on Monte Carlo dose calculations proved to be appropriate for target sizes down to 0,8 cm (diameter). The accurate positioning of the setup is an important aspect for small field irradiation. |
