Doctor of Philosophy
Centre for Medical Radiation Physics, Engineering Physics
Wong, Jeannie Hsiu Ding, Implementation of silicon based dosimeters, the dose magnifying glass and magic plate for the dosimetry of modulated radiation therapy, Doctor of Philosophy thesis, Centre for Medical Radiation Physics, Engineering Physics, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3348
New cutting edge radiation therapy techniques such as Intensity Modulated Radiation Therapy (IMRT), Stereotactic Radiosurgery (SRS), Helical TomoTherapy and most recently Volumetric Modulated Arc Therapy (VMAT) produce radiation dose maps with high dose modulation and tight gradients between the high and low dose region. Difficulties in the dosimetric verification of these new complex treatment methods using existing dosimeters have led to the need for a new generation of fast responding real time dosimeters with submillimetre precision. This thesis describes two detector systems based on silicon substrates that were developed to address this need. The first detector system was a silicon strip detector called the “Dose Magnifying Glass” (DMG). It consisted of 128 detectors spaced 0.2 mm apart. It was coupled to a TERA ASIC chip that enabled simultaneous readout of multiple channels at high temporal resolution. The first part of the thesis involved investigation of the basic characteristics of the DMG followed by its application in the dosimetric verification of IMRT, SRS and Helical TomoTherapy treatment deliveries. The high spatial resolution of this device was ideal for the measurement of high dose gradients in IMRT and small fields encountered in SRS. When compared with film dosimetry, DMG measurements showed agreements within 3% for a SRS treatment plan. The DMG was also successfully employed as an independent quality assurance tool for the verification of helical Tomotherapy machine binary MLC leaf parameters. The second detector was a two dimensional array detector so named the “Magic Plate” (MP). The diode was based on epitaxial technology and has a very thin sensitive volume of 50 μm. The MP comprised of 11 × 11 epitaxial diodes mounted on a 0.6 mm thick Kapton substrate. This detector was designed to be used either as a transmission detector or to measure dose distributions in a solid water phantom. Preliminary testing of the MP in a clinical IMRT treatment delivery was carried out. The MP measurements demonstrated good agreement (>80%) with conventional EBT2 film dosimetry and with treatment planning system predicted dose distributions using 3%/3mm gamma criteria.