Doctor of Philosophy
Centre for Medical Radiation Physics
In recent years new radiotherapy systems have emerged that are utilized for small tumour treatments with improvements to enable improved dose coverage of the target. The treatment is accurate but still would benefit from a real-time treatment monitoring with high spatial and temporal resolution sampling for regular quality assurance (QA). A monolithic silicon diode array, the Magic Plate 512 (MP512) was developed as a potential candidate for such QA. The detector was design for use as an in-phantom 2D dosimeter and 2D transmission mode detector for real-time dose measurements.
The first part of this thesis evaluates the radiation response of the Magic Plate and the impact of an air gap immediately above the MP512. This air gap is then optimize for using the MP512 for small field dosimetry in both photon and electron fields. The output factor (OF), percentage depth dose (PDD) and enhanced dynamic wedge (EDW) beam profiles were measured as a part of these studies. The optimized air gap is then taken into account in the later chapters that focus on in-phantom dosimetry using the MP512. MP512 response reduces with increasing air gap above the detector. The OF measured with MP512 with air gaps of 0.5 mm and 1.2 mm show a good agreement with OF measured with the EBT3 film (within ±2%) and MOSkin for 6 MV and 10 MV, respectively. Similar results were observed for the PDD measurement. The EDW dose profile matched well with the EBT3 for the air gap of 0.5 mm within ±2% (1 standard deviation) for all wedge angles. The PDD measured by electron beams demonstrated no significant effect of the air gap size above MP512 for all energies.
The second part of this thesis demonstrates the use of MP512T as a transmission detector. The influence of operating the MP512T in transmission mode (TM) on the surface dose of a phantom was evaluated as a function of different field sizes and distances from the solid water phantom to transmission detector (Dsd). For all Dsd and all field sizes, the MP512T led to the surface dose increasing by between 5% and 25% when in the beam, depending on the configuration. The transmission factor of the MP512T ranged from 1.020 to 0.9950 for all measured Dsd and field sizes.
The last part of this thesis showed the correlation of transmission mode response (TM) and dose mode response (DM) of the Magic Plate512 (MP512T) for different detector to surface distances (Dsd) and treatment field sizes. The measured correlation between TM and DM was then employed to predict the dose at dmax for regular fields, and intensity modulated fields. The calculated dose for regular fields of 1 x 1cm2 and 4 x 4cm2 fell in the range of [-2.18% and +1.95%] compared to the measured dose. For the calculated IMRT planar dose at dmax and gamma criteria of 3%/3mm and 2%/2mm pass rates of 98.14%/90.5% and 97.22%/93.8% were found when compared to the dose predicted by the TPS for Dsd 4 and 24cm, respectively. Good agreement was also observed for these gamma criteria when comparing TM measurements taken at Dsd 4 and 24cm with EBT3 yielding pass rates of 96.89%/92% and 97.53%/93.8%, respectively.
The thesis therefore ultimately demonstrates that the dose in the phantom can be calculated based on TM measurements and these data represent the first step in the development of real-time high spatial resolution 3D dose reconstruction technique based on TM measurements from the MP512.
Utitsarn, Kananan, Monolithic Silicon Transmission Pixelate Detector for Small Field Dosimetry, Doctor of Philosophy thesis, Centre for Medical Radiation Physics, University of Wollongong, 2017. https://ro.uow.edu.au/theses1/338