Master of Science (MS)
Physics and Astronomy
Purpose: To determine the dosimetric accuracy of a dose reconstruction method used for verification of helical tomotherapy delivery for three different clinical sites. Methods and Materials: A delivery verification and dose reconstruction method has been applied to helical tomotherapy treatment plans of three different treatment sites (head & neck, prostate and lung). Treatment plans were generated on a cylindrical measurement phantom (TomoPhantom) using contours, prescriptions and planning objectives taken from clinical patient plans of the three sites. Film and ion chamber measurements were made for each plan with and without intentional changes in the machine output [-4% to 4%] or leaf open times [-30 ms to +30 ms] to the planned delivery. A TomoTherapy delivery verification tool uses pulse-by-pulse machine CT detector and transmission ion chamber data, extracted at the conclusion of each delivery, to determine the incident energy fluence delivered for each projection. Dose reconstruction was calculated by simulating the delivered energy fluence onto the planning CT. The reconstructed doses were compared with both the measured and planned dose distributions. Results: Measured dose variations for repeated daily deliveries were small, typically within 2%. Greatest differences between the measured dose and planned dose occurred when intentional changes in leaf open times (±30 ms) were made to the delivery. Measured doses from all deliveries were well predicted by the dose reconstruction method, which demonstrated agreement for point doses to within 2%. The dose reconstruction method also demonstrated acceptable agreement with the film dose measurements for all three plans. Comparison of film versus reconstructed dose for all cases showed that over 90% of a selected region of interest had a gamma index of less than 1. Conclusion: The method of dose reconstruction based on machine detector data can account for daily variations in the delivered dose due to machine error. Dosimetric accuracy for the method is acceptable within clinical standards.
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Hesston, Ricky, "Dosimetric evaluation of a delivery verification and dose reconstruction method for helical tomotherapy" (2009). LSU Master's Theses. 2491.
John P. Gibbons