Sophisticated test objects for the quality assurance of optical computed tomography scanners.

Optical computed tomography (CT), in conjunction with radiochromic gels and plastics, shows great potential for radiation therapy dose verification in 3D. However, an effective quality assurance (QA) regime for the various scanners currently available still remains to be developed. We show how the favourable properties of the PRESAGE® radiochromic polymer may be exploited to create highly sophisticated QA phantoms. Five 60 mm diameter cylindrical PRESAGE® samples were irradiated using the x-ray microbeam radiation therapy facility on the ID-17 biomedical beamline at the European Synchrotron Radiation Facility. Samples were then imaged on the University of Surrey parallel-beam optical CT scanner. The sample irradiations were designed to allow a variety of tests to be performed, including assessments of linearity, modulation transfer function (three independent measurements), geometric distortion and the effect of treatment fractionation. It is clear that, although the synchrotron method produces extremely high-quality test objects, it is not practical on a routine basis, because of its reliance on a highly specialized radiation source. Hence, we investigated a second possibility: three PRESAGE® samples were illuminated with ultraviolet light of wavelength 365 nm, using cheap masks created by laser-printing patterns onto overhead projector acetate sheets. There was good correlation between optical densities measured by the CT scanner and the expected UV 'dose' delivered. The results are encouraging and a proposal is made for a scanner test regime based on calibrated and well-characterized PRESAGE® samples.