Development of an Optical Detector for Daily Quality Assurance of the Multileaf Collimator of the Elekta Unity MR-Linac
Abstract
Purpose
To develop an optical detector for daily quality assurance (QA) of the 80 multileaf collimator (MLC) leaf pairs of an Elekta Unity MR-LINAC.
Methods
A prototype optical detector, developed in collaboration with Medscint Inc. (QC, CA), was designed for MLC leaf positioning measurements in an MR-LINAC environment. The detector consists of four plastic scintillating fibers (BCF-12, 1.0-mm diameter, 61-cm length) coupled to optical fibers and read out using a Hyperscint RP-200 spectrometer (Medscint Inc.). The fibers, spaced 5-mm apart, were mounted on a 67-cm wide acrylic plate on the Elekta QA platform at isocenter height, aligned perpendicular to the MLC motion, with four embedded radiopaque beads enabling localization using the on-board electronic portal imaging device (EPID). A dedicated MLC sequence comprising three irradiation configurations per tested leaf was developed within the EPID field of view. Two configurations involved opening a single leaf in each bank (Y1 or Y2), with the field edge centered on the midpoint of the four fibers. A third configuration with both banks opened provided a reference field. Spectral and temporal integrals were normalized to this reference and fitted using a four-parameter sigmoidal model as a function of fiber position. The fitted inflection point was used to determine the position of each tested MLC leaf. Corresponding positions were also extracted from EPID images using the zero-crossing of the Laplacian of a Gaussian.
Results
Irradiations of 5 MU per MLC configuration were sufficient to measure leaf positions, with a standard deviation below 0.35~mm for shifts up to 3~mm. Mean differences were below 1~mm relative to EPID and 0.3~mm relative to planned positions. The detector reliably detected a 1.5~mm introduced positioning error and corrected device misalignments without loss of accuracy.
Conclusion
This optical scintillation detection enables fast, submillimetric MLC position verification suitable for daily QA on Elekta Unity.