Purpose: To demonstrate a novel light-to-radiation coincidence tool with adjustable field edges.
Methods: We designed and constructed a simple, mechanical light-to-radiation (L2R) coincidence measuring device that uses adjustable light field markers to determine the light field coincidence with EPID. This L2R edge tool consists of four sliders, one for each side of a square field, that are 5cm away from the crosshair centre with ±0.5cm adjustment to allow for alignment of the device to the field edge, rather than adjusting the linac jaws. X- and Y-jaw line markers have thickness approximately equal to the corresponding light penumbra of that jaw (designed for Varian Truebeam Linear Accelerator) such that sliders are adjusted to cover the penumbra, reducing error in visual interpretation of the light field edge. A computer vision algorithm identifies ball bearings that have been placed 1.00cm from each light field marker, finds the radiation field edges as 50% amplitude points from central profiles, and calculates L2R offsets for each jaw. The L2R tool was commissioned by comparing results to film and assessing inter-observer variability.
Results: For the same linac, light-to-radiation field offsets for the L2R tool vs film were -0.3mm vs -0.2mm for X1, -0.9mm vs -0.4mm for X2, 0.2mm vs -0.1mm for Y1, and -0.2mm vs 0.0mm for Y2. The horizontal and vertical radiation field sizes were mean ± standard deviation (10.01±0.03)cm and (9.96±0.02)cm for film, respectively, and (10.01±0.00)cm and (9.96±0.01)cm for the L2R edge tool, respectively. The standard deviation of L2R offsets between four individuals on the same linac ranged from 0.1–0.3mm.
Conclusion: The L2R edge tool is simple and practical, and measures L2R field coincidence quickly, reproducibly, and in good agreement with film.
Quality Control, Linear Accelerator, Commissioning
TH- External Beam- Photons: Quality Assurance - Linear accelerator