Direct aperture optimization: A turnkey solution for step‐and‐shoot IMRT

TLDR

Step‑and‑shoot IMRT plans have traditionally been generated by optimizing intensity maps for each beam angle. This study introduces an automated planning system that bypasses intensity optimization and directly optimizes aperture shapes and weights. The authors developed a direct aperture optimization algorithm that enforces MLC delivery constraints within the optimization, simultaneously optimizing leaf settings and aperture intensities via simulated annealing, and tested it on patient cases using EGS4/BEAM Monte Carlo dose calculations. Direct aperture optimization produced highly conformal step‑and‑shoot plans with only three to five apertures per beam, significantly reducing beam segments and monitor units while preserving full IMRT dosimetric benefits.

Abstract

IMRT treatment plans for step‐and‐shoot delivery have traditionally been produced through the optimization of intensity distributions (or maps) for each beam angle. The optimization step is followed by the application of a leaf‐sequencing algorithm that translates each intensity map into a set of deliverable aperture shapes. In this article, we introduce an automated planning system in which we bypass the traditional intensity optimization, and instead directly optimize the shapes and the weights of the apertures. We call this approach “direct aperture optimization.” This technique allows the user to specify the maximum number of apertures per beam direction, and hence provides significant control over the complexity of the treatment delivery. This is possible because the machine dependent delivery constraints imposed by the MLC are enforced within the aperture optimization algorithm rather than in a separate leaf‐sequencing step. The leaf settings and the aperture intensities are optimized simultaneously using a simulated annealing algorithm. We have tested direct aperture optimization on a variety of patient cases using the EGS4/BEAM Monte Carlo package for our dose calculation engine. The results demonstrate that direct aperture optimization can produce highly conformal step‐and‐shoot treatment plans using only three to five apertures per beam direction. As compared with traditional optimization strategies, our studies demonstrate that direct aperture optimization can result in a significant reduction in both the number of beam segments and the number of monitor units. Direct aperture optimization therefore produces highly efficient treatment deliveries that maintain the full dosimetric benefits of IMRT.