Master of Science (MS)
Physics and Astronomy
Purpose: Electron skin collimation (eSC) sharpens the penumbra, allowing electron radiotherapy of cancers while minimizing dose to adjacent normal tissue, e.g., protecting the eye in treating eyelid and nose cancers. This study assessed the clinical potential for milled brass eSCs to replace manually-fabricated lead or Cerrobend eSCs. Methods: Aim one was to measure the influence of eSC edge angle on the dose distributions, which were measured under edges of lead eSCs for combinations of three bevel (0°,±10°) and five gantry (0°,±3°,±5°) angles for a 6-MeV beam. Aim two was to validate the dimensional conformity of three machined brass eSCs, one designed for a cylindrical phantom and two for an anthropomorphic head phantom. eSC shapes were compared using megavoltage computed tomography (MVCT) of brass eSCs, kilovoltage computed tomography (kVCT) of wax replicas, and physical measurements. Aim three compared dose distributions in the cylindrical phantom for a brass eSC (≈ 7×9 cm2) with those for lead and Cerrobend eSCs at 16-MeV. All dose distributions were the average of three radiochromic EBT2 film measurements. Results: Aim one showed all combinations of (0°,±10°) bevel and (0°,±3°,±5°) gantry angles to have insignificant effect on penumbral width (≈ 0.5 mm). Increased dose from scatter, as great as 11% near the edge, was clinically insignificant because of low surface dose for 6 MeV beams and insignificant change in R90. Aim two showed MVCT, kVCT, and physical measurements useful for quality assurance. Also, it revealed that distal eSC surfaces matched designs (≤ 2 mm), that thicknesses deviated ≤ 1.5 mm from design, and that two of three eSCs had aperture edges focused ≈20 cm above the eSC generating errors up to 3 mm. Aim three showed the brass eSC to have comparable radiation leakage, penumbral width, field width, and coverage of distal 90% dose contour, but increased scatter dose (3-10%). Conclusions: Results indicated that machined brass eSC holds good potential for replacing lead or Cerrobend eSCs. Prior to clinical utilization, two recommendations are (1) to eliminate the cause of incorrect divergence in the aperture edges and (2) to determine the bevel angle that minimizes scatter dose from brass eSC edge.
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Posey, Ryan Kenneth, "Utilization of machined skin collimation for electron therapy" (2012). LSU Master's Theses. 457.