ESTRO 2021 Abstract Book

S467

ESTRO 2021

Lawrenceville, USA

Purpose or Objective A novel parallel-beam optical CT scanner, utilizing fiber optic taper for collimated images, was developed for fast, high resolution, and accurate readout of 3D dosimeters. In this study, the performance of this newly designed optical CT scanner was characterized and evaluated by comparison with the dose distributions from Eclipse and EBT3, as well as 3D readout from a single-laser beam optical CT scanner in our lab. Materials and Methods The optical CT scanner incorporates a telecentric illuminator with parallel red-light beam (630 nm ± 5nm), an aquarium filled with optical matching fluid to the refractive index of the 3D PRESAGE phantom, a fiber optic taper for collimation of transmitted light, and a CCD camera. Stray light due to scatter, reflections, and refractions are removed due to the collimation effect of the fiber optic taper. The effect of collimation on the 3D dose measurement was studied. System linearity, spatial resolution, MTF, SNR, and image distortion tests were performed. 3-D dosimetry comparisons, with PRESAGE dosimeters, between the fast parallel-beam optical CT scanner with fiber optic taper and a single-beam optical raster scanner were performed for square field, conformal arc, IMRT fields, and multiple-lesion VMAT arcs. Based on our previous 3D dosimetry studies, the single-beam optical CT scanner, modified from a commercial OCTOPUS scanner, was considered the “gold standard” for 3D studies. The 2D dose distributions generated by the fiber optic CT scanner were also compared to Eclipse planning and EBT3 results.

Results With collimated images from fiber optic taper the highest spatial resolution is estimated to be 0.07 mm with MTF 10%. The scanning time for a complete 3-D dataset acquisition is less than 10 minutes. For 2-D dose distribution comparison, with small-field conformal arcs, the gamma passing rate (2%/2mm criteria, 10% dose threshold) was 100% between the fast fiber optic CT scanner and EBT3; and 99% between the fiber optic scanner and the single-beam scanner. Additional dose distribution comparisons were also made between optic fiber scanning of PRESAGE measurements and Eclipse calculations along axial, coronal, and sagittal central planes for IMRT H&N 7 fields and 2 arcs VMAT. The gamma passing rates were between 97.1 % to 99.4 % for 3%/2mm gamma criteria. All the dose distributions from the fiber optic CT scanner were reconstructed directly from the 2D projected images without any correction for stray light. The stray light was removed almost completely through the collimation effect of the fiber optic taper in this scanner.

Conclusion The newly designed parallel-beam optical CT scanner, utilizing fiber optic taper for collimation of transmitted lights, has been successfully implemented in our lab. It can provide a fast, high resolution, and accurate dose