Optical Imaging Provides Rapid Verification of Static Small Beams, Radiosurgery and VMAT Plans with millimeter resolution.
PURPOSE
We demonstrate the feasibility of optical imaging as a quality assurance tool for static small beamlets, and pre-treatment verification tool for radiosurgery and VMAT plans.


METHODS
Small static beams and clinical Volumetric-modulated arc therapy (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 treatment planning system (TPS) data. 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 was also tested. The sensitivity to common delivery errors was also compared against a commercial 2.5D diode array dosimeter.


RESULTS
A spatial resolution of 1 mm 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 5 mm 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 1 mm and gantry angle errors of 1°.


CONCLUSIONS
Optical imaging provides ample spatial resolution for imaging small beams. The ability to faithfully image down to 20 MU of 5 mm, 6 MV beamlets prove the ability to perform quality assurance for each control point within dynamic plans. The technique is sensitive to small offsets errors in gantry angles and Multi-leaf collimator (MLC) leaf positions, and at certain scenario it exhibits higher sensitivity than a commercial 2.5D diode array. This article is protected by copyright. All rights reserved.