Volume 29, Issue 1, January 2002
Index of content:
- PH. D. THESES ABSTRACTS
Accommodating practical constraints for intensity modulated radiation therapy by means of compensators29(2002); http://dx.doi.org/10.1118/1.1429625View Description Hide Description
The thesis deals with the practical implementation of intensity modulated radiation therapy(IMRT) generated by means of patient specific metal compensators. An elaborate comparison between several compensator-machining techniques, with respect to their suitability for production within a hospital workshop, is presented. The limitations associated with the selected compensator manufacturing technique are identified and implemented as constraints in an existing inverse treatment-planning algorithm. In order to obtain the profile of a compensator, which produces a desired intensity distribution, inverse modeling of the radiation attenuation within the compensator is required. Two novel and independent approaches, based on deconvolution and system identification, are proposed to accomplish this. To compare the approach with the “rival” state of the art beam modulation technique, a theoretical and experimental examination of the modulated fields generated by manufactured compensators and multileaf collimators is presented. This comparison focused on the achievable resolution of the intensity modulated beams in lateral and longitudinal directions. To take into account the characteristics of a clinical environment the suitability of the most common commercially available treatment couch systems for IMRTtreatments is studied. An original rule based advisory system is developed to alert the operator of any potential collision of the beam with the movable supporting structures of the treatment couch. The system is capable of finding alternative positions for the supporting frames and, if necessary, can suggest alternative beam directions. Finally, a head and neck phantom is designed for geldosimetry to assess IMRTtreatment delivery techniques. The phantom is based on a simplistic but realistic design and contains the main anatomical features.
29(2002); http://dx.doi.org/10.1118/1.1429626View Description Hide Description
This thesis investigates the validity of inverse algorithms and delivery techniques in dynamic intensity modulated radiation therapy(IMRT) with the view toward clinical implementation. A dosimetric verification technique used to monitor the dynamic beam delivery was introduced with design considerations and clinical evaluation of various quality assurance (QA) phantoms, facilitating the measurement of dynamic IMRTdose distributions and conversion of photon fluence to machine deliverable monitor units. Benchmark tests using thermoluminescent dosimeters(TLDs), solid state detectors, and films were carried out in realistic clinical examples. This was to demonstrate the precision and degree of accuracy of the proposed techniques both in vitro and in vivo systems using canine subjects with paraspinal tumors. The results of the irradiation of canine subjects indicated that the goal of using IMRT to irradiate target volumes with sparing critical structures was met. However, the measure of physiological response found, was incapable of providing a more accurate measure of the dosedelivered. Quantitative analysis on the influence of systematic and random field perturbations in highly conformal fields with emphasis on the dosimetric outcome is also presented in this thesis. Two sources of uncertainties deemed important in dynamic therapy of photon beams were studied. These were caused by inaccuracies in velocity and in position of both multileaf collimator(MLC) and backup diaphragms. This was attributed to some degree to the tolerance levels or mechanical constraints of beam defining parameters. These errors were simulated using a Gaussian function with standard deviation of ±1.0 mm to assess changes in dose distribution. Based on the results of this study, QA procedures which are unique to the delivery of dynamic beams were proposed.
29(2002); http://dx.doi.org/10.1118/1.1429627View Description Hide Description
Positron emission tomography(PET)images include a significant scatter component. The presence of the scatter manifests itself as a loss of spatial resolution and as an apparent migration of activity from hot to cold regions. Monte Carlo(MC) based approaches have proved to be very useful in simulating the image-forming process in the PET scanner. However, fully 3D MC simulations are computationally too intensive to be applied in clinical routine. Consequently, many efforts have been undertaken to develop approximate scatter correction techniques for PET. Scatter correction is generally performed prior to image reconstruction using an appropriate model of the scatter process. These models require estimates of the correct emission and attenuation distribution in the imaged object. The problem is that these estimates are computed from measured data and, therefore, already contain scattered events. The purpose of this work was to overcome this problem by incorporating scatter characteristics directly into the process of iterative image reconstruction. This was achieved by an optimized implementation of the Single Scatter Simulation (SSS) algorithm resulting in a significant speed-up of the scatter estimation procedure (from about 10 min to 30 s). The computationally improved SSS algorithm was then included in the forward projection step of maximum likelihood image reconstruction. Results obtained from phantom measurements demonstrate that this approach leads to a better estimation of the scatter component than can be obtained by a simple sequential data processing strategy.