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Polymer Gel and Magnetic Resonance Relaxometry Based 3D Dose Monitoring
Hira, Ranjit Singh
Sinha, UshaAnderson, MattKassegne, SamuelSinha, Shantanu
xv, 82 pages : illustrations (some colored).
Recent advances in radiation therapy physics have enabled sophisticated treatment plans that conform accurately to the target volume while sparing normal tissue. The complex three- dimensional treatment planning requires dose verification (dosimetry) techniques that can validate with high spatial resolution in 3D dose distribution with accuracy and precision. Routinely used dosimetry techniques include ionization chambers which can map very accurately but are restricted to a single plane or 2D film based techniques. Gel dosimetry can be considered as 3D integrating dosimeters that offer high spatial resolution, precision and accuracy for the verification of a large range of treatment plans. However, gel dosimetry has not been evaluated for a wide range of treatment plans, especially using more advanced radiation therapy units such as Stereotactic Radiosurgery. In addition gel dose readouts are performed with Magnetic Resonance Imaging sequences that are extremely long, precluding routine usage of this technique in a clinical setting. This thesis is focused on establishing gel dosimetry as a viable method for clinical use to validate a wide variety of treatment plans. A new fast MR pulse sequence (stepped TE multi-shot Echo Planar Imaging) is proposed for T2 (spin-spin relaxation) relaxometry studies and evaluated against the conventional multiple spin echo sequence. Three different radiation dose treatment plans using Intensity Modulated Radiation Therapy (IMRT), Volumetric Modulated Arc Therapy (VMAT) and Stereotactic Radio Surgery (SRS) systems were evaluated with the gel dosimeter. Since the dose calibration curves did not yield accurate absolute values of dose, several methods for scaling are discussed. Gel dosimetry provided accurate isodose contour lines for the three treatment plans investigated in this thesis. Calibration using small vials did not yield accurate absolute values of dose, however the internal scaling schemes provided accurate absolute values. The dose maps from the gel dosimetry agreed to within +-5% (acceptable range) of the treatment plan dose in the targeted volume of interest as well as in the peripheral dose fall off regions. Future plans include internal dose calibration methods to increase the accuracy of absolute dose quantification.
Includes bibliographical references (pages 80-82).
Master of Science (M.S.) San Diego State University, 2014
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