Ir-192 HDR Source Parameter Validation Using Multichannel Gafchromic Film

Abstract

Currently, Ir-192 HDR treatment planning system commissioning is only based on a single absolute measurement of source activity supplemented by validation of TG-43 parameters via dose point spot checks without independent verification that the planned distribution corresponds to the actual delivered dose. Reports in the literature indicate a need for more thorough tests to evaluate end-to-end accuracy of the treatment planning system. Gafchromic film has the potential for widespread clinical use to verify multiple source parameters due to its convenience for application without requiring a processor, excellent spatial resolution imperative in brachytherapy, and reported accuracy in external beam quality assurance. The purpose on this work is to present a methodology to validate the planned dose distribution for a single HDR dwell position as part of initial treatment planning commissioning and each source exchange to ensure that there are no unexpected changes in the source itself. The procedure presented independently calculates the one-dimensional dose perpendicular to the source, creates a robust calibration curve, and then compares a planned two-dimensional dose distribution with measured film dosimetry. A sheet of Gafchromic EBT3 film positioned in contact with a delivery catheter surrounded by water-equivalent scatter material is irradiated with an Ir-192 Nucletron HDR source. Dwell time is selected to be slightly below the film saturation density at the normalization point 5.0 mm from the source center. The film is digitized with an Epson 10000 XL color scanner. Film analysis is performed with MatLab imaging toolbox. Transmission values for the green channel, previously determined to have the greatest range and contrast, was converted into optical density (OD) as a function of perpendicular distance from the center of the calculated line source center. A corresponding theoretical dose curve was calculated using the TG43 formalism. The calculated dose in 0.1 mm increments was fitted to the measured OD in the 5-25 mm range to a sixth order polynomial. A comparable single source plan was created using the Oncentra treatment planning system. The 3D dose matrix was exported to a Varian Eclipse work station for convenient extraction of the 2D coronal dose plane with predetermined origin and scale at the level of the source for MatLab analysis. The Oncentra and measured film dose planes were registered at their corresponding origins as determined by lateral and vertical symmetry. The 2D dose matrices were individually scaled to 0.1 mm per pixel. The OD calibration curve consisting of 200 points was statistically valid, robust, and in agreement with Oncentra planned dose within +/- 1.5 % in the clinical range. A sixth order polynomial was necessary to accurately fit the film response throughout the usable film density range. Agreement between film and planned dose matrices was calculated at each corresponding position in the 2D dose plane as the absolute difference between the plan and measured doses. Areas near the source (5 mm lateral and 10 mm vertical to the source center) were excluded. Over 97% of the points had a dose agreement difference of less than 3% at exact corresponding points, which is an improvement over previously-published results using a gamma metric of 3% AND 3 mm. Figure below shows the agreement difference distribution. For clarity, only one side of the symmetrical image is displayed. The area within the white lines is the excluded region near the source. An accurate and robust calibration curve using the green channel of Gafchromic film can be easily created for use in validating HDR treatment planning systems. A single dwell position treatment plan verified with film dosimetry can serve to document the accuracy of all the theoretical model parameters. The methodology presented will be useful in implementing more comprehensive quality assurance on treatment planning systems and may even be extended in future work to verify patient-specific dose distributions.