Abstract
This study demonstrates the degradation in image quality, and subsequent dose evaluation inaccuracies, that are encountered when an optical-CT system reconstructs an image slice of a gel dosimeter containing an opaque implant, and evaluates the feasibility of a simple correction method to improve the accuracy of radiotherapy dose distribution measurements under these circumstances. MATLAB was used to create a number of different virtual phantoms and treatment plans along with their synthetic projections and reconstructed data sets. The results have illustrated that accurately evaluating 3D gel dose distributions in the vicinity of high-Z interfaces is not possible using the filtered back projection method, without correction, as there are serious artefacts throughout the dose volume that are induced by the missing ray-sum data. Equivalent artefacts were present in physical measurements of irradiated PAGAT gel containers when read by an optical-CT system. An interpolation correction performed prior to reconstruction via the filtered back projection algorithm has been shown to significantly improve dose evaluation accuracy to within approximately 15 mm of the opacity. With careful placement of the implant within the gel sample, and use of the linear interpolation method described in this study, there is the potential for more accurate optical CT imaging of gels containing opaque objects.
Highlights
Gel dosimeters show potential for measuring complex radiation dose distributions, in radiotherapy [1,2,3,4,5,6,7,8,9,10]
A 50% downstream dose depletion caused by a high-density implant has been simulated; the increased doses caused by lateral-scatter and backscatter were not considered in these simulations
For high-density implant dosimetry, characterising the downstream dose depletion is of paramount importance
Summary
Gel dosimeters show potential for measuring complex radiation dose distributions, in radiotherapy [1,2,3,4,5,6,7,8,9,10]. Among the potential applications of gel dosimeters in radiotherapy are those in which the irradiated portion of the gel dosimeter surrounds an imbedded object, such as a metallic implant [11,12,13], head and neck phantoms [14], inserts for brachytherapy measurements [15], or implanted scintillators [16]. In these cases, options for readout of gel dosimeters are limited; there is significant potential for artefacts in MRI and x-ray CT, and missing data in optical computed tomography (optical-CT) due to opacity of the object. A number of different reconstruction methods to reduce metal artefacts in x-ray CT have been proposed [18, 19] showing mixed results
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