Volume 31, Issue 3, March 2004
Index of content:
- PH. D. THESES ABSTRACTS
31(2004); http://dx.doi.org/10.1118/1.1644511View Description Hide Description
The American Association of Physicists in Medicine (AAPM) Task Group 43 report (TG-43) provides a dosimetry protocol for brachytherapydose calculations, and results for some sources. Gold 198 seeds days, have been used for brachytherapy treatments including brain, eye, and prostate tumors. However, the National Institute of Standards and Technology (NIST) does not have a standard for seeds as for and seeds, and the TG-43 report gives no data for For these reasons and with the conversion of treatment planning systems to TG-43 based methods, both Monte Carlo calculations (MCNP 4C) and thermoluminescent dosimeters (TLD’s) are used in this work to determine TG-43 data. MCNP is also used to model a standard well chamber’s response for a seed relative to both an seed and a seed. After activation, seed activity is determined using both a well chamber and a HPGe detector. The geometric variation in dose is measured using TLD’s in a solid water phantom. The results for air kerma strength, per unit apparent activity, are 2.06 (MCNP) and 2.09 (measured) The former is identical to the AAPM TG-32 report results. The dose rate constant results, Λ, are 1.12 (MCNP) and 1.10 (measured) The radial dose function, anisotropy function, and anisotropy factor, are given. The anisotropy constant values are 0.973 (MCNP) and 0.994 (measured) and are consistent with both source geometry and photon energy. The calculated response of a Sun Nuclear ionization chamber, model 1008, demonstrates that to convert known and seed activities to equivalent” activities, they should be multiplied by 1.95 and 1.49, respectively.
31(2004); http://dx.doi.org/10.1118/1.1644512View Description Hide Description
The research work contained in this manuscript relates to the accurate calculation of the dosedelivered to patients during intensity modulated radiation therapy and focuses primarily on radiotherapy of the head and neck. However, the methods utilized herein are applicable to other clinical sites. This investigation aims to address the need to develop more accurate modeling of three-dimensional dose distributions in patients, arising from the use of intensity modulated beams. This work has been carried out by using and developing computational techniques, primarily Monte Carlo simulation codes, specifically designed for medical applications. The accurate simulation of radiationsources, beam modulators, patient irradiation, and dosimetric verification systems has been successfully accomplished. Computations, calibrated in terms of absolute dose were extensively verified versus dosimetric data. Calculated and measured dose profiles agreed within 2% for open fields and 3% for modulated fields. The developed MC model for the generation of IMRT fluence maps was found to be accurate at the 2%/2 mm level when compared to portaldosimetry data. The retrospective application of the developed techniques to a number of clinically relevant cases has also been explored. A number of methods for the analysis, comparison, and verification of three-dimensional dose distributions have been suggested and implemented. The system developed in this work and presented in the form of a comprehensive computational toolbox has been proven to be robust and accurate for Monte Carlo dose calculations for the verification of intensity modulated radiotherapy plans.
A study of multi-modality imaging for three-dimensional radiotherapy treatment planning of brain tumors31(2004); http://dx.doi.org/10.1118/1.1645631View Description Hide Description
Integration of information from multiple imaging modalities like CT/MRI/SPECT/PET helps in management of cancer, from tumor detection to assessment of treatment efficacy since each imaging modality provides unique information about the tumor and normal structures. This Ph.D. study has been aimed at evaluation of multi-modality imaging in target localization and radiation treatment planning in patients with malignant glioma undergoing radiotherapy. Variations of gross tumor and the edema volumes delineated by a radiologist on CT and MRI scans were addressed. These scans were registered and transferred to the treatment planning system (TPS) using Windows based automatic image registrationsoftware developed in house using AIR routines (Roger Woods, UCLA). QA tests were designed and tested for the 3D automatic target expansion algorithm of the treatment planning system. The influences of variation of target volumes in CT and MRIimages on cumulative dose distribution were also analyzed.Software for quantitative analysis of DVHs has been developed (for Isis-3D TPS) and results showed the importance of incorporation of targets from multiple imaging modalities. A novel approach to study the radiation-induced changes in normal appearing white matter (NAWM) using T2 and magnetization transfer ratio (MTR) maps was developed. The study demonstrated dose-dependent changes of the MTR values and reversal of MTR values seen for doses However, the T2 maps did not show any dose depended changes within the time period studied. The following in-house software tools were developed to accomplish various tasks of the thesis work: (a) Windows based automatic image registration (AIR) of multi-modality images suitable for radiotherapy planning, (b) to preprocess “bitmap” images from film scanner suitable for AIR software, (c) to transfer registered “tiff” images into the Isis-3D planning system, (d) quantitative analysis of DVH curves generated by the Isis-3D system, and (e) quantitative analysis of sequential MRI scans to generate and analyze T2, MTR, and ADC maps. These software tools, along with source codes, can be freely supplied upon request to the author.
31(2004); http://dx.doi.org/10.1118/1.1645632View Description Hide Description
The thesis describes the development and implementation of a novel method of deliveringintensity modulated radiation therapy(IMRT) that provides greater accuracy and spatial resolution than currently available methods. Through improvements in multileaf collimator(MLC) based fluence generation, a dose distribution may be generated that conforms more closely to the tumour target volume. Healthy tissue surrounding the target volume will therefore receive less dose, reducing the probability of side effects and allowing the physician to increase the prescribed tumordose(dose escalation). As a preamble to the description of the IMRTdelivery method a new model for evaluating the spatial resolution capabilities of dosedelivery techniques is presented. Flexibility and complexity in patient treatment due to advances in radiotherapy techniques necessitates a simple method for evaluating spatial resolution capabilities of the dosedelivery device. The model is based on linear systems theory and is analogous to methods used to describe resolution degradation in imaging systems. The spatial resolution capabilities of different delivery components can be quantified separately, providing a simple method for comparing different treatment machine characteristics. Also, the model provides the ability to evaluate spatial resolution changes independent of the tumor that is being treated, providing a means of comparing delivery techniques that is not biased by the characteristics of any particular treatment volume. MLC based IMRT techniques are well established but suffer several physical limitations. Dosimetricspatial resolution is limited by the MLC leaf width, interleaf leakage and tongue-and-groove effects degrade dosimetric accuracy and the range of leaf motion limits the maximum deliverablefield size. Based on observations from the linear systems model it is hypothesized that, by rotating the entire MLC between each sub-field, improvements will be obtained in spatial resolution,dosimetric accuracy and maximum deliverablefield size. To generate arbitrary fluence maps in this way, a series of unique algorithms were developed that are capable of determining the necessary rotated MLC segments. These IMRT fields may be delivered statically (with the collimator rotating to a new position in between sub-fields) or dynamically (with the collimator rotating and leaves moving simultaneously during irradiation). A full description of the rotational leaf motion algorithms is provided. An analysis of the rotating leaf motion calculation algorithms with focus on radiation efficiency, the range of collimator rotation, and number of segments is provided. The mechanical and radiation producing characteristics of standard linear accelerators under collimator rotation conditions are also investigated. The technique is evaluated by characterizing the ability of the algorithms to generate rotating leaf sequences for desired fluence maps. Comparisons are also made between our method and conventional sliding window and step-and-shoot techniques. Results show improvements in spatial resolution, reduced interleaf effects, and maximum deliverablefield size over conventional techniques. Clinical application of these enhancements can be realized immediately with static rotational delivery although improved control of the MLC will be required for dynamic delivery.