Year

2011

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering, Centre for Medical Radiation Physics

Abstract

The concept of \spectroscopic dosimetry" was investigated and applied in the de- velopment of the urethral mini-dosimetry system, an intra-operative dosimeter de- signed to provide real time dose measurements along the urethra during low dose rate prostate brachytherapy, complementing current planning and treatment techniques. Spectroscopic dosimetry involves the summing of measured events corresponding to characteristic photo peaks of the treatment isotopes. As low dose rate brachyther- apy seeds emit low energy photons ( 27keV for I-125), energy loss for the treatment radiation in tissue is mainly due to the photoelectric e ect. The Compton cross- section is signi cant, but energy loss per scattering event is minimal. Photo peaks within acquired spectra are attenuated in intensity only, with minimal shift in peak energy. The number of counts within the photo peaks of a spectrum, measured with a silicon detector, is proportional to the dose rate at the point of measurement.

Monte Carlo simulations, performed using EGSnrc version 4, showed spectro- scopic dosimetry is accurate as predicting the measured dose rate to within 4.2% up to 4cm of I-125 based low dose rate brachytherapy seeds. Silicon based radiation detectors operating in spectroscopy mode may be used for spectroscopic dosimetry as spectroscopic dosimetry does not require the detector to be tissue equivalent. Only the ratio of dose deposited in the detector medium to dose deposited in tissue needs to be constant. Monte Carlo simulations also demonstrated silicon detectors are suitable for use in spectroscopic dosimetry.

The urethral mini-dosimetry system was constructed using a silicon detector within a probe of length 40cm, connected to a spectroscopic ampli cation and data acquisition system, and underwent rigorous testing in preparation for clinical use. The testing consisted of several in phantom measurements, studying many aspects of the device functionality. Phantom testing concluded the urethral mini-dosimeter probe may be constructed with angular isotropy to within ±5%, and may measure the combined dose rate from multiple seeds in a three dimensional array to within ±5%.

The first generation urethral mini-dosimetry system has shown promise as a tool for quality assurance during low dose rate prostate brachytherapy. The second generation urethral mini-dosimetry system is being constructed to improve on the measurement capabilities of the rst generation model. Clinical trials at St George Cancer Care Centre, Australia, and Memorial Sloan Kettering Cancer centre, USA, will proceed upon the manufacturing of the new mini-dosimeter probes associated with the second generation urethral mini-dosimetry system.

The urethral mini-dosimetry system will provide a new method of low dose rate prostate brachytherapy dosimetry and seed localisation. Based on a silicon detector in spectroscopy mode, the urethral mini-dosimetry system will provide a new ap- proach to quality assurance in low dose rate brachytherapy, with real time dosimetry of accuracy within 5%. The concept of \spectroscopic dosimetry" may also be ex- tended to multiple detector systems, such as strip detectors and pixelated detectors, to provide spatial information as well as dosimetry within brachytherapy.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.