Master of Science (MS)
Physics and Astronomy
Purpose: To perform a secondary dose calculation for intensity modulated radiotherapy (IMRT) or volumetric modulated arc therapy (VMAT) plans to a point on or off axis within 2% using open field data. Methods: An independent dose calculation algorithm has been developed for complex fields with multiple segments. The algorithm subdivides dose into the contributions from each opposing leaf pair for a given multileaf collimator (MLC) configuration. Leaf pair dose is determined by drawing four rectangular fields based on leaf positions, which are symmetric about the point of calculation. Superposition of these fields yields the dose from the leaf pair to the point. VMAT plans are approximated by a static MLC configuration at four degree intervals. The algorithm requires standard open field data (e.g., head and phantom scatter factors, Scps and tissue phantom ratios, TPRs), and the MLC control point information. Calculations were done with additional measured small field output factors down to a 1.5x1.5-cm² field. Algorithm doses to the isocenter or center of the planning target volume (PTV) were compared with heterogeneous Pinnacle calculations of a series of prostate, head and neck, and chest wall treatment plans. Delivery techniques included fixed gantry IMRT and VMAT. Results: Good agreement was obtained between doses calculated by the algorithm and the Pinnacle3 treatment planning system. Percent errors were -0.2% ± 3.8% (mean and 95% confidence interval) for algorithm calculations. Systematic offsets were observed as a function of calculation site, with prostate doses being underestimated and chest wall doses being overestimated. Errors are likely the result of patient geometry deviations from the infinite slab, flat phantom assumption of monitor unit calculations. Conclusion: Results demonstrate that clinically acceptable agreement is obtained using this method. Further improvement could be made with more accurate heterogeneity correction factors and/or a better estimation of small field output factors.
Document Availability at the Time of Submission
Release the entire work immediately for access worldwide.
Watts, Adam Michael, "A secondary monitor unit calculation algorithm using superposition of symmetric, open fields for IMRT plans" (2013). LSU Master's Theses. 1742.