Master of Science (MS)
Physics and Astronomy
Purpose: To develop a system for measuring 2D dose distributions in the cranium and to use this system to evaluate the accuracy of coplanar conformal therapy using ExacTrac image guidance. Methods: Techniques were developed to measure dose distributions in each principal plane using a CIRS anthropomorphic head phantom with a custom internal film cassette. Sections of EDR2 film were cut, processed, and digitized using custom templates. Spatial and dosimetric accuracy and precision of the film system was assessed. BrainScan was used to plan a coplanar-beam treatment conforming to irradiate a 2-cm diameter x 2-cm cylindrical target. Prior to delivery phantom misalignments were imposed in combinations of Â±8-mm offsets in each of the principal directions. ExacTrac X-ray correction was applied until the phantom was within an acceptance criteria of 1mm-1Â° (first two measurement sessions) or 0.4mm-0.4Âº (last two measurement sessions). Measured dose distributions on film were registered to the treatment plan and compared. Results: Alignment errors(displacement between midpoints of planned and measured 70% isodose contours), were 0.48Â±0.40, -0.20Â±0.44 and 0.45Â±0.43mm along the Posterior-Anterior, Right-Left and Superior-Inferior directions, respectively, using acceptance criteria of 1mm-1Â°, and 0.72Â±0.18, 0.12Â±0.18 and -0.14Â±0.34mm, respectively, using acceptance criteria, 0.4mm-0.4Â°. Positional errors of the 80% isodose line were -0.36Â±0.43, 0.38Â±0.38, and 0.03Â±0.44mm, on the Posterior, Right, and Inferior edges of each profile, respectively, and 0.68 Â±0.40, -0.14 Â±0.38, and 0.88Â±0.40mm for the Anterior, Left, and Superior edges, respectively for the 1mm-1Â° criteria. Using the 0.4mm-0.4Â° criteria, errors were -0.35Â±0.23, 0.31Â± 0.18, and 0.72Â±0.27mm on the Posterior, Left and Inferior portion of each profile respectively and 1.12Â±0.18, 0.22Â±0.23, and 0.35Â±0.33mm for the Anterior, Left and Superior portions, respectively. Data ranged approximately two standard deviations about the mean. Winston-Lutz tests with errors of approximately 0.5mm correlated with increased Anterior error in three measurement sessions. Conclusions: The ExacTrac system is capable of achieving sub-mm(2Ïƒ) accuracy given the alignment errors were small(<0.3mm) when Winston-Lutz results were small(<0.2mm), indicating pre-treatment corrections could be made.
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Vinci, Justin, "Accuracy of cranial coplanar beam therapy with BrainLAB ExacTrac image guidance" (2007). LSU Master's Theses. 453.