Semester of Graduation
Master of Science (MS)
Physics & Astronomy
Purpose: Bolus electron conformal therapy (BECT) provides effective radiation treatment for superficial cancers and other diseases close to the skin surface, but can have as great as a 30% planning target volume (PTV) dose heterogeneity due to scattering from the irregular proximal bolus surface. Intensity modulated (IM) BECT can improve PTV dose homogeneity, but is not currently available. This study fabricated patient-specific passive intensity modulators and validated their delivering planned dose distributions calculated by a modified pencil beam redefinition algorithm (PBRA).
Methods: Two test-patterns and four patient-specific intensity modulators were designed, fabricated, and tested. Dose plans were generated using a research version of p.d (.decimal LLC, Sanford, FL), which contained an intensity modulation operator. Dose distributions under intensity modulators were measured using a water phantom and scanning diode. The PBRA was modified to calculate dose in the presence of island blocks (tungsten pins of varying diameters) embedded in a low-density, machinable foam contained within an electron cutout.
Results: Dose under island blocks with axes parallel to central axis was greater than expected, believed due to electrons scattered from island blocks, hence island blocks with axes along rays diverging from the virtual source were recommended. The PBRA modeled machineable foam by shifting R90 0.1 cm shallower and scaling σθx by 1.5, calculating dose distributions under foam with an accuracy equal to that without foam; however, foam increased the penumbra indicating it beneficial to reduce its thickness (g×cm2). PBRA modifications for island blocks yielded doses within 3% of measurements for IRF >75%, indicating the need to model scatter from and into island blocks for lower IRFs. For all four patient-specific intensity modulators, measured doses were within 3%/3mm of calculated doses for ≥99.5% of points having dose >10%, proving the hypothesis.
Conclusions: Results showed that patient intensity modulators could deliver dose (fluence) within 3%/3mm of that planned, indicating the PBRA was sufficiently accurate for the patients studied and that .decimal can fabricate intensity modulators capable of delivering planned dose distributions. Comparison of dose distributions measured with a dose matrix for additional patient plans having greater intensity modulation is needed to establish future QA criteria.
Hilliard, Elizabeth Noel, "Verification and Evaluation of a Passive Intensity Modulation Device for Bolus Electron Conformal Therapy" (2018). LSU Master's Theses. 4784.