Design of a Passive Intensity Modulation Device for Bolus Electron Conformal Therapy

Abstract

Purpose: To develop a process that can design an island block matrix that produces an intensity distribution (70-100%), which acceptably matches one planned for an intensity modulated (IM) bolus electron conformal therapy (ECT) patient. The intensity modulator concept is that electrons laterally scatter behind hexagonally-packed, small-diameter island blocks such that local intensity equals the fractional unblocked area. Methods: A pencil beam algorithm (PBA) was used to calculate the modulated electron intensity created by varying diameter (d) of island blocks in a hexagonal array (separation r). Accuracy of the PBA model was assessed by comparing with Monte Carlo (MC) calculations. PBA calculations determined acceptability of (r, d) values for achieving clinical intensity reduction factors (70% Results: PBA and MC calculations agreed within &177;5%. At 103 cm SSD PBA results showed r < 0.5 cm and 0.75 cm at 0.5 cm and 2.0 cm depths, respectively, acceptable for 7-20 MeV electrons; larger r values were acceptable for lower energies. Although larger r require fewer blocks, smaller r decreased the distance to transition to the desired IRF, helping achieve intensity distributions with sharp gradients. The Modulator Generator required <5sec (r=0.5cm) to produce clinically-acceptable distributions for the buccal mucosa patient (>99% of points within 3% of planned intensity). Conclusions: The PBA model was sufficient to study the impact of island block parameters (r, d) on achieving desired IRFs for differing conditions (energy, SSD, depth); however, PBA-MC agreement should be improved for patient use. The primary objective was achieved; electron intensity modulators comprised of island blocks of variable diameter can be designed to deliver a desired intensity distribution of clinical complexity (70-100%) with an accuracy of &177;3% for 95% of modulated points.