Master of Science (MS)


Physics and Astronomy

Document Type



Background: State-of-the-art radiotherapy medical records include reliable estimates of the therapeutic radiation but are known to underestimate the stray radiation exposures by 40% away from the treatment field. Most commonly, stray radiation exposures are reconstructed using empirical formulas and/or lookup tables containing machine-specific dose measurements. The purpose of this study was to develop a physics-based model to calculate exposures to the whole body of patients who receive external beam photon radiotherapy. Methods: We developed a physics-based analytic algorithm to predict absorbed dose from therapeutic, scatter, and leakage radiation. The model includes separate terms to characterize photon production, attenuation, and scattering in the treatment unit as well as attenuation and scatter of radiation within the phantom. It was developed using measurements of total absorbed dose in a water-box phantom from a 6 MV medical linear accelerator and was validated against measured profiles in water using several clinically representative treatment fields. Results: Our dose algorithm reproduces the measured dose profiles in water from 1.5 to 10 cm in depth and 35 cm off-axis distance. At least 90% of predicted doses agreed within 10% or 3 mm of measured absorbed doses for positions where those doses were greater than 5% of therapeutic dose, and within 2 mGy of stray dose per Gy of therapeutic dose or 10 mm of measured doses at other locations. Computation times for 10 million dose points within a phantom were less than 6.5 minutes. Conclusions: The results suggest that it is feasible to use a physics-based model to accurately and quickly predict whole body exposures from radiation therapy. A potentially important advantage of a physics-based approach, such as the algorithm proposed in this work, is that the model is inherently more readily adaptable to a wide variety of treatment units and treatment techniques than models based on empirical formulae or machine specific lookup-tables.



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Committee Chair

Newhauser, Wayne