AbstractsPhysics

The scope of this project was to measure dose profiles using Gafchromic® XR-QA2 films when clinical protocols were used to scan a PMMA phantom with CT scanners and kV Cone Beam Computed Tomography (CBCT) systems integrated into linear accelerators. Estimated volume CT dose index CTDIvol values based on measured dose profiles were compared to tabulated data in order to assess the reproducibility and accuracy of this method in clinical use. The feasibility study of the radiochromic film-based dosimetry system included an evaluation of the film response as a function of the effective photon energy over the energy range used in radiology and estimation of the precision of the CTDI measurements during helical (on CT scanner) and cone beam CT acquisitions (on linear accelerator).Energy dependence of the Gafchromic® XR-QA2 film was tested over the imaging energy range covered by multiple commercially available CT scanners and on-board imaging (OBI) devices on linear accelerators. A General Electric LightSpeed® LS 16 radiotherapy simulator was used for this purpose. The effective energy of multiple beams was estimated via HVL measurements, and the device output was obtained following the AAPM TG-61 protocol. Strips of film were irradiated in air to known air kerma values ([Kair]air) with the x-ray tube of the CT scanner in static mode. The reflectance change of the film prior and after irradiation was assessed using an in-house Matlab code with the TIFF images of the films scanned by an Epson® Expression 10000XL flatbed document scanner. Calibration curves for each beam quality were created to model the response of the film in the [Kair]air range up to100 mGy. Pixel values were read out after applying a low filter kernel to the extracted red channel from the images. Responses of the film for the same dose values were then compared.Film strips were sandwiched between PMMA rods cut in half and placed into both the peripheral and central holes of a CTDI phantom. Three sets of films were irradiated under the same scanning parameters, and the procedure was repeated for several clinical scanning protocols on each imaging device. The change in the reflectance of the film was converted into [K_air ]_air using calibration curves and subsequently converted into dose to water (D_w) using mass energy absorption coefficients. Finally, D_w profiles were averaged to calculate CTDIvol values. Averaged CTDIvol values from three measurements (CTDIvol|XR-QA2) were compared to the corresponding tabulated CTDIvol values provided by the manufacturer (CTDIvol|tabulated). Dispersion of the data was used to assess reproducibility of the dosimetric system. Measured to tabulated CTDI values ratio was used to assess accuracy.The relative variation of the film response was determined to be inversely proportional to the absorbed dose. The maximum absolute variation was observed at 30 mGy over the studied effective energy range. The observed variation diminishes up to 50 % as dose decreases to 5 mGy, and up to 75 % as dose increases to 100 mGy. The…