Volume 26, Issue 3, March 1999
Index of content:
- PH. D. THESES ABSTRACTS
26(1999); http://dx.doi.org/10.1118/1.598540View Description Hide Description
The verification of complex dose distributions produced using novel techniques in radiotherapy requires measurements in three dimensions with high spatial resolution. Recent developments in polymergeldosimetry employing MRI suggest this may be the best available method. In this work the properties of BANG (bis, acrylamide, nitrogen, and gelatin) polymergel were investigated. The gel was found to be tissue equivalent and its response to absorbed dose reproducible to within ±4% and linear up to 10–12 Gy. The response of the gel was also found to be independent of energy and dose rate, but dependent on oxygen contamination and gel temperature during MR imaging. The application of BANG gel in different areas of radiotherapy was investigated. In brachytherapy a comparison of the relative dose rate distributions between gel, calculation, and TLDs agreed to within ±5%. The measureddose distributions from complex brachytherapy and multifield external beam irradiations agreed well with the distribution produced by the Helax-TMS planning system. For external beam irradiation the depth dosesmeasured in the gel showed good agreement with ion chamber measurements to within ±3%. The gel was also used to verify the absorbed dose distribution produced by intensity modulated beam techniques using compensators. The results agreed well with film dosimetry. BANG gel was also found to be an excellent candidate for dynamic measurements and measurements in areas with restricted access. A novel technique for measurements in boron neutron capture therapy using boron-loaded gel is also described. It was concluded that gel can be used to verify complex dose distributions. [Copies of the thesis are available from Main Library, University of Leicester, University Road, Leicester, LE1 7RH, England.]
26(1999); http://dx.doi.org/10.1118/1.598541View Description Hide Description
This thesis deals with the implementation of intensity-modulated radiation therapy into the clinic and a method of quality assurance which can be used on each intensity-modulated beam prior to treatment. The first component of the thesis was to write a step-and-shoot leaf sequence algorithm to control the multileaf collimator(MLC) fitted to our Clinac 2300 C/D linear accelerator. Our algorithm takes into account the MLC transmission, MLC penumbra, and change in scatter conditions with field size and is also slightly more efficient than other published step-and-shoot type algorithms. The second (and major) component of the thesis was to investigate the use of a portal imaging device for dosimetric verification purposes. We show that an electronic portal imaging device of the scanning liquid ionization chamber type yields images which, once calibrated from a previously determined calibration curve, provide highly precise planar maps of the incident dose rate. For verification of an intensity-modulated x-ray beam delivered in the segmented approach with a MLC, a portal image is acquired for each subfield of the leaf sequence. Subsequent to their calibration, the images are multiplied by their respective associated monitor unit settings, and summed to produce a planar dose distribution at the measurement depth in phantom. The excellent agreement of our portal imager measurements with calculations of our treatment planning system and data from a one-dimensional beam profiler attests to the usefulness of this method for the planar verification of intensity-modulated fields produced in the segmented approach on a computerized linear accelerator equipped with a multileaf collimator.
26(1999); http://dx.doi.org/10.1118/1.598542View Description Hide Description
dosimetry was examined by calculative and experimental means. The Monte Carlo N-Particle (MCNP) transport code was used in a distributed computing environment (PVM) to determine neutrondose and neutronenergyspectrum from in a variety of clinically relevant materials. A Maxwellianspectrum was used to model neutron emissions in these materials. Mixed-field dosimetry of applicator tube (AT) sources was measured using tissue-equivalent ion chambers and a miniature GM counter to formulate a dosimetry protocol. Neutrondose was determined using the AAPM TG-43 dosimetry protocol. Results demonstrate the overwhelming dependence of dosimetry on the source geometry and no significant neutron attenuation by the source or encapsulation. Gold foils and TLDs were used to measure thermal neutron flux near AT sources and compared with MCNP results. The fast neutronenergyspectrum did not change markedly at greater distances from the AT source. Calculations of moderated neutronenergyspectrum with various loadings of and were performed, in addition to analysis of neutron capture therapy dosimetry. Radiological concerns such as personnel exposure and shielding of emissions were examined. Feasibility of a high specific-activity HDR source was investigated through radiochemical and metallurgical studies using stand-ins such as Tb, Gd, and Issues such as capsule burst strength due to helium production for a variety of proposed HDR sources were addressed. At least 1 mg of was necessary for a HDR source.