A method for obtaining three-dimensional measurements of HDR brachytherapy dose distributions using Fricke gel dosimeters and optical computed tomography.

This study aimed to develop a method for performing accurate, high-resolution, three-dimensional (3D) Fricke gel dosimetry measurements of high dose rate (HDR) brachytherapy dose distributions using optical computed tomography (CT). A multi-needle brachytherapy gel phantom was purpose-built to contain four stainless-steel brachytherapy needles and a sample of Fricke Xylenol gel. A Paris-style HDR brachytherapy treatment was planned and delivered to the gel, which was then read out using a novel optical CT scanning method; all the brachytherapy needles were removed prior to scanning and replaced with a refractive index matched fluid. The removal of the stainless-steel needles during pre- and post-irradiation scanning minimised the potential for artefacts caused by missing ray-sum data. Results showed good agreement between measured and calculated doses (within 1%) at all positions greater than 0.1 cm from each needle. This study demonstrated that 3D Fricke gel phantoms may be valuable tools in verifying HDR brachytherapy treatments. The phantom construction and optical CT scanning method proposed in this work has the potential to enable routine quality assurance measurements of complex HDR brachytherapy treatment deliveries via accurate and detailed three-dimensional dose measurements.

Technical Note: Dose distributions in the vicinity of high-density implants using 3D gel dosimeters.

PURPOSE: In this work, we develop a methodology for using Fricke gel dosimeters for dose distribution measurements surrounding high-density implants which circumvents artifact production by removing the obstruction during imaging. METHODS: Custom 3D printed molds were used to set cavities in Fricke gel phantoms to allow for the suspension of high-density implants in different geometries. This allowed for the metal valve extracted from a temporary tissue expander to be suspended during irradiation, and removed during optical-CT scanning. RESULTS: The removal of the metal implant and subsequent backfilling of the remaining cavity with optically matched fluid prior to dose evaluation enables accurate optical-CT scanning of the gel dosimeters. Results have shown very good agreement between measured and calculated doses within 2 mm from the surface of the implant. Slight deviations are present within 1 mm of the interface. CONCLUSIONS: Artifacts in the form of radial streaking, cold spots, and hot spots were all reduced using this technique, enabling the broader and more accurate use of optical-CT for the imaging of gels containing opaque objects.

Angular dependence of the response of the nanoDot OSLD system for measurements at depth in clinical megavoltage beams.

PURPOSE: The purpose of this investigation was to assess the angular dependence of a commercial optically stimulated luminescence dosimeter (OSLD) dosimetry system in MV x-ray beams at depths beyond d(max) and to find ways to mitigate this dependence for measurements in phantoms. METHODS: Two special holders were designed which allow a dosimeter to be rotated around the center of its sensitive volume. The dosimeter's sensitive volume is a disk, 5 mm in diameter and 0.2 mm thick. The first holder rotates the disk in the traditional way. It positions the disk perpendicular to the beam (gantry pointing to the floor) in the initial position (0°). When the holder is rotated the angle of the disk towards the beam increases until the disk is parallel with the beam ("edge on," 90°). This is referred to as Setup 1. The second holder offers a new, alternative measurement position. It positions the disk parallel to the beam for all angles while rotating around its center (Setup 2). Measurements with five to ten dosimeters per point were carried out for 6 MV at 3 and 10 cm depth. Monte Carlo simulations using GEANT4 were performed to simulate the response of the active detector material for several angles. Detector and housing were simulated in detail based on microCT data and communications with the manufacturer. Various material compositions and an all-water geometry were considered. RESULTS: For the traditional Setup 1 the response of the OSLD dropped on average by 1.4% ± 0.7% (measurement) and 2.1% ± 0.3% (Monte Carlo simulation) for the 90° orientation compared to 0°. Monte Carlo simulations also showed a strong dependence of the effect on the composition of the sensitive layer. Assuming the layer to completely consist of the active material (Al2O3) results in a 7% drop in response for 90° compared to 0°. Assuming the layer to be completely water, results in a flat response within the simulation uncertainty of about 1%. For the new Setup 2, measurements and Monte Carlo simulations found the angular dependence of the dosimeter to be below 1% and within the measurement uncertainty. CONCLUSIONS: The dosimeter system exhibits a small angular dependence of approximately 2% which needs to be considered for measurements involving other than normal incident beams angles. This applies in particular to clinical in vivo measurements where the orientation of the dosimeter is dictated by clinical circumstances and cannot be optimized as otherwise suggested here. When measuring in a phantom, the proposed new setup should be considered. It changes the orientation of the dosimeter so that a coplanar beam arrangement always hits the disk shaped detector material from the thin side and thereby reduces the angular dependence of the response to within the measurement uncertainty of about 1%. This improvement makes the dosimeter more attractive for clinical measurements with multiple coplanar beams in phantoms, as the overall measurement uncertainty is reduced. Similarly, phantom based postal audits can transition from the traditional TLD to the more accurate and convenient OSLD.