Abstract
The ongoing development of easily accessible, fast optical readout tools promises to remove one of the barriers to acceptance of gel dosimetry as a viable tool in cancer clinics. This paper describes the characterization of a number of basic properties of the Vista™ cone beam CCD-based optical scanner, which can obtain high resolution reconstructed data in less than 20 min total imaging and reconstruction time. The suitability of a filtered back projection cone beam reconstruction algorithm is established for optically absorbing dosimeters using this scanner configuration. The system was then shown to be capable of imaging an optically absorbing media-filled 1 L polyethylene terephthalate (PETE) jar dosimeter to a reconstructed voxel resolution of 0.5 × 0.5 × 0.5 mm3. At this resolution, more than 60% of the imaged volume in the dosimeter exhibits minimal spatial distortion, a measurement accuracy of 3–4% and the mean to standard deviation signal-to-noise ratio greater than 100 over an optical absorption range of 0.06–0.18 cm−1. An inter-day scan precision of 1% was demonstrated near the upper end of this range. Absorption measurements show evidence of stray light perturbation causing artifacts in the data, which if better managed would improve the accuracy of optical readout. Cone beam optical attenuation measurements of scattering dosimeters, on the other hand, are nonlinearly affected by angled scatter stray light. Scatter perturbation leads to significant cupping artifacts and other inaccuracies that greatly limit the readout of scattering polymer gel dosimeters with cone beam optical CT.
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General scientific summary. Modern radiation therapy has been a rapidly evolving field over the last decade, involving increasingly sophisticated treatment planning and delivery options. Conventional one- and two-dimensional dosimetry tools such as ion chambers, diodes, arrays, film, etc. available in the clinic are well suited to regular quality assurance. However, there are particular circumstances where a three-dimensional (3D) dosimeter would be useful for spatial dose verification of sophisticated new dose deliveries. Gel dosimetry is a promising candidate for 3D dose measurements of complex new treatment techniques and for process quality assurance, but has not been employed clinically, for a number of reasons. In this work, we complete a basic characterization of a fast cone beam optical CT scanner for 3D readout of optically absorbing gel dosimeters. Focused development of this optical readout tool promises to improve the accessibility and ease of use of gel dosimetry in the clinical environment.