Room: Exhibit Hall | Forum 3
Purpose: To evaluate the accuracy of a modified pencil beam redefinition algorithm (PBRA) calculating dose to a water phantom for a test pattern intensity modulator for intensity-modulated bolus electron conformal therapy (IM-BECT).
Methods: Electron beam intensity can be modulated using an intensity modulator consisting of a machineable foam slab embedded with island blocks (tungsten pins of varying diameters on a hexagonal grid), which fill the electron aperture. The PBRA code in a research version of p.d BolusECT (.decimal LLC, Sanford, FL) was modified to calculate dose in the presence of a passive, electron intensity modulator. Island blocks of differing diameter were selected to deliver a desired, modulated beam intensity (fluence). Dose under a test-pattern intensity modulator was measured using a water phantom and scanning diode.
Results: The modified PBRA modeled the machineable foam by shifting Râ‚‰â‚€ by -0.1 cm to account for a small energy loss and scaling Ïƒ(Ï‘x) by a factor of 1.5 to account for the increased angular spread of the beam. With these modifications, dose was calculated under the foam (without island blocks) with accuracy equal to that without foam. In the presence of the test-pattern intensity modulator the PBRA calculated doses within 3% or 3 mm of the measured dose for beam intensities greater than 75%, but had greater errors for lower intensities (57-75%).
Conclusion: Results showed that the increased angular spread due to the machineable foam broadened the penumbra sufficiently so that it was recommended that its thickness (g*cmÂ²) be decreased as much as practical. The most significant differences between calculated and measured doses occurred downstream of island blocks with the largest 0.4-cm diameter (57% intensity). Future Monte Carlo studies are planned to determine minor modifications to the PBRA to account for scattering into and out of the island block edges.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the National Cancer Institute of the National Institutes of Health (NIH) under Award Number R41CA199838.