Optical imaging provides rapid verification of static small beams, radiosurgery, and VMAT plans with millimeter resolution.

Abstract

We demonstrate the feasibility of optical imaging as a quality assurance tool for static small beamlets, and pretreatment verification tool for radiosurgery and volumetric-modulated arc therapy (VMAT) plans. Small static beams and clinical VMAT plans were simulated in a treatment planning system (TPS) and delivered to a cylindrical tank filled with water-based liquid scintillator. Emission was imaged using a blue-sensitive, intensified CMOS camera time-gated to the linac pulses. For static beams, percentage depth and cross beam profiles of projected intensity distribution were compared to TPS data. Two-dimensional (2D) gamma analysis was performed on all clinical plans, and the technique was tested for sensitivity against common errors (multileaf collimator position, gantry angle) by inducing deliberate errors in the VMAT plans control points. The technique's detection limits for spatial resolution and the smallest number of control points that could be imaged reliably were also tested. The sensitivity to common delivery errors was also compared against a commercial 2.5D diode array dosimeter. A spatial resolution of 1mm was achieved with our imaging setup. The optical projected percentage depth intensity profiles agreed to within 2% relative to the TPS data for small static square beams (5, 10, and 50mm2 ). For projected cross beam profiles, a gamma pass rate >99% was achieved for a 3%/1mm criteria. All clinical plans passed the 3%/3mm criteria with >95% passing rate. A static 5mm beam with 20 Monitor Units could be measured with an average percent difference of 5.5±3% relative to the TPS. The technique was sensitive to multileaf collimator errors down to 1mm and gantry angle errors of 1°. Optical imaging provides ample spatial resolution for imaging small beams. The ability to faithfully image down to 20MU of 5mm, 6MV beamlets prove the ability to perform quality assurance for each control point within dynamic plans. The technique is sensitive to small offset errors in gantry angles and multileaf collimator (MLC) leaf positions, and at certain scenario, it exhibits higher sensitivity than a commercial 2.5D diode array.