Master of Science
University of Wollongong. Department of Engineering Physics
Wilkinson, Dean, Commissioning of an In Vivo Dosimetry system for high dose rate gynaecological brachytherapy, Master of Science thesis, University of Wollongong. Department of Engineering Physics, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3277
This thesis encompasses experimental work performed at The Canberra Hospital in collaboration with the Centre for Medical Radiation Physics, University of Wollongong, in the field of high dose rate (HDR) brachytherapy. The purpose of this research was to develop an accurate and reliable in vivo dosimetry program that could be used to measure the dose delivered to organs at risk, such as the rectum and bladder, during HDR gynaecological brachytherapy. The current method of determining dose to these organs is based on calculation of a point dose by the treatment planning system (TPS). This calculation is limited in accuracy due to the inability of the TPS to consider heterogeneities in the dose calculation process. The implementation of an alternate dosimetry system independent to the TPS would be a useful tool for brachytherapy.
The instruments chosen for measuring dose were a PTW five-diode array for use in the rectum; a PTW single diode probe for the bladder; and a uniquely designed Metal Oxide Semiconductor Field Effect Transistor (MOSFET), known as a MOSkinTM, developed at the Centre for Medical Radiation Physics. Diodes and MOSFETs are commonly used in radiotherapy because of their desirable characteristics for accurate dose measurement. A thimble ionisation chamber traceable to a primary standards laboratory was used as a reference dosimeter for the calibration of these devices.
To develop a better understanding of how these dosimeters may respond when used in brachytherapy, simulations were carried out using the EGSnrc Monte Carlo code. An 192Ir source was modelled to emulate the micro Selectron ‘Classic’ HDR brachytherapy source used at The Canberra Hospital. The dose distribution and spectrum for this source was analysed in various media to determine how these characteristics changed with distance from the source. The Monte Carlo simulations showed a significant change in the 192Ir photon spectrum with distance in water. The change in spectrum was attributed to the increasing portion of low energy scattered photons with distance from the source. Further investigation was also performed to determine the difference in dose deposited ina small silicon voxel compared to a voxel of water at various distances from the source.
This gave an indication as to how the higher atomic number of the semiconductor detectors would affect their response at lower photon energies. The implications of these effects, i.e. the increase cross section for photoelectric absorption in silicon at low photon energies, helped determine the most appropriate distance to calibrate the dosimeters.
After completing the Monte Carlo simulations, the inherent characteristics of the rectum diode probe, bladder diode probe, and MOSkinTM were investigated. The characteristics measured for each dosimeter included the energy dependence, angular dependence, linearity, long-term and short-term reproducibility and temperature dependence. By determining the inherent characteristics of each device, an uncertainty in measurement was calculated for each dosimeter. It was also found that the ionisation chamber displayed a poor spatial resolution when attempting to measure at distances close to the source. This was attributed to the significant dose gradient across the large air volume of the chamber. As a result, the ionisation chamber was deemed to be insufficient for calibrating the dosimeters close to the source.
The final outcome of this work was the development of an appropriate calibration procedure for the diode probes and MOSkinTM that was sufficiently accurate and easy to reproduce. Two different phantom designs were evaluated: the first being a fixed cylindrical jig comprised of PMMA that could simultaneously calibrate all three dosimeters against an ionisation chamber at 8 cm from the source; and the second a perspex slab phantom allowing the dosimeters to be placed individually at any distance between 2.3 and 8 cm from the source. Each phantom was imaged on a CT scanner and reconstructed in the TPS so that the doses measured by each of the dosimeters could be compared with those calculated by the TPS. Under full scattering conditions the dosimeters displayed good agreement with the TPS at large distances from the source but as the dosimeters were moved closer to the source a discrepancy between measured and calculated dose was observed. While calibrating at a non-clinically relevant depth of 8 cm was undesirable, the accuracy was far greater than attempting to calibrate the dosimeters close to the source where a large dose gradient turns small errors in positioning into significant errors in dose.