Semester of Graduation

Summer 2021

Degree

Master of Science (MS)

Department

Department of Physics and Astronomy

Document Type

Thesis

Abstract

Purpose: Bolus electron conformal therapy (BECT) benefits treatment of the post-mastectomy chest wall, head and neck, paraspinal muscles, and extremities. Patient dose heterogeneities caused by bolus can be reduced through intensity modulation (IM) across the incident electron beam. This requires passive radiotherapy intensity modulation for electrons (PRIME) devices, which utilize tungsten pins (island blocks) imbedded in machinable foam. IM-BECT treatment planning requires accurate dose calculation using the pencil beam redefinition dose algorithm (PBRA). Currently, the PBRA models island blocks as perfect collimators. This work explores models to account for electrons scattering into and out of the sides of island blocks.

Methods: A pencil beam model was used to compute a pin diameter (dIS) that corrected for in-scatter for a given beam energy and physical (nominal) pin diameter (dnom). Percent depth-dose and off-axis profiles in water were measured for each of 36 combinations of four PRIME devices, each with uniform pin diameters (0.0, 0.158, 0.273, 0.352 cm), three beam energies (7, 13, 20 MeV), and three SSDs (100, 105, 110 cm). Similarly, out-scattered electrons were modeled by modifying pin diameter. An initial model based on Monte Carlo calculating a pin’s effect on the dose distribution failed to improve accuracy of the PBRA-calculated dose relative to that calculated using dnom. Therefore, a second model was developed, which estimated out-scatter as the difference between measured and in-scatter-adjusted PBRA-calculated dose distributions. Then, a single out-scatter dose correction was determined using a least squares minimization, from which a new pin diameter (dIS+OS) was determined.

Results: A table of dIS+OS values was generated as a function of beam energy, SSD, and dnom. For the 27 combinations, passing rates (3%/3 mm) for the PBRA-calculated versus measured dose distributions were determined; compared to those using dnom, those using dIS+OS improved for 11, remained the same for 13, and worsened for 3.

Conclusions: The hypothesis was only conditionally met. However, upon study completion it was discovered that PRIME devices used for measurement were incorrectly fabricated. Future measurements and data analysis with correctly fabricated devices will refine scatter corrections, possibly making both Monte Carlo and measurement methods acceptable.

Committee Chair

Pitcher, Garrett

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