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PURPOSE: To investigate the accuracy of the absorbed dose measured with Gafchromic EBT2 film in low-energy photon radiation fieldsMethods: Six EBT2 film (lot # F06110901) pieces (1cm2 ) per dose were exposed to x-rays of 50 kV, 80 kV, 120 kV and 60Co gamma rays from a Leksell Gamma Knife at dose values from 50 mGy to 100 Gy. The x-ray beams were calibrated following the AAPMTG-61 protocol using ionization chambers calibrated at NIST or Wisconsin University depending on the beam quality, while the 60Co gamma was calibrated in water using MD-V2-55 film. Each film piece was scanned once using a HP Scanjet 7650 document flatbed scanner in transmission mode, 48-bit color at 300 dpi spatial-resolution. The data analysis was made through the ImageJ. The measured light intensity for the red channel with its associate standard deviation was used to evaluate the netOD and its standard combined uncertainty. The absorbed dose as a function of the netOD was fitted using the logistic model and the relative combined uncertainties were evaluated for each energy photon beam. RESULTS: EBT2 film response curve depends on the low-energy photons and the degree of energy-dependence is a function of absorbed dose. The absorbed dose relative combined uncertainty as a function of the absorbed dose indicates that the minimum absorbed dose limit is also energy dependent. Lower is the energy photon; more accurate is the measurement at low dose value. This can be explain by the fact that comparing to high energy photons, low energy photons can produce locally enough ionization density to create more color centre in the same film area. CONCLUSIONS: Minimum absorbed dose limit of Gafchromic EBT2 films were found to be energy dependent. The response curve depends on the low-energy photons and the degree of energy-dependence is a function of absorbed dose This work is partially supported by DGAPA-UNAM grant IN102610 and Conacyt Mexico grant 127409.
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PURPOSE: Besides flattening-filter-free beams, Varian TureBeam Linac also has conventional flattened photon beams. In our facility, we have TrueBeam, Trilogy and iX machines from Varian; they all have same energy specifications: 6 and 10 MV photon beams, as well as 6, 9, 12, 16 and 20 MeV electron beams. This study is to compare the photon and electron beams dosimeter parameters among the three machines. METHODS: Beam data (including PDDs, inline and crossline profiles at various field sizes and various depths) were collected using Sun Nuclear Dosimetry 3D Scanner with nominal 100 cm SSD setup. These data were post processed using Sun Nuclear Dosimetry software, including normalization, interpolation and smoothing. The ion chambers used for scanning are IBA CC13. RESULTS: Photon beams: The percentage depth doses with field sizes of 4×4, 6×6, 10×10, 15×15, 20×20, 30×30 and 40×40 cm × cm of 6 MV and 10 MV photon beams from the three machines are very close. Compared with Varian Golden Beam Data, the maximal variation of PDDs at depths of 5, 10, 15, 20, and 30 cm is 1.0%, with mean value 0.6% and standard deviation 0.28% for 6 MV; for 10 MV beams, they are 2.0% (at depth of 30 cm), 0.9%, and 0.48% respectively. Also, the three machines have very similar beam profiles; the profiles' shoulder, penumbra and umbra match well for both inline and crossline beam profiles at various field sizes and various depths.Electron beams: As compared the percentage surface doses (0.5 mm from the surface), dmax, R90, R80, R50, and R30 of electron beams with energy of 6, 9, 12, 16, and 20 MeV at 10 cm cone, the electron beams of TrueBeam and iX are almost identical. CONCLUSIONS: The 6 and 10 MV photon beam data of TrueBeam, Trilogy, and iX have a same variation range when comparing with Varian Golden Beam Data.
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PURPOSE: Magnetic Resonance Spectroscopy (MRS) of the prostate is not used in radiotherapy departments on a regular basis due to a number of issues. The indication and severity of prostate cancer is related to the presence of choline in the prostate, in particular, the ratio of choline (plus creatine) to citrate. In-vivo data supports this theory only marginally but lacks strong correlation with biopsy data. The situation is further complicated by the lack of precise spatial information in biopsy, variation of magnetic susceptibility, and spatial dependence of MRS data on the distance from the endo-rectal coil. The latter also cause low signal-to-noise ratio (SNR). We intend to understand how the level of metabolite concentrations and spatial dependences determine what is observed in MRS. METHODS: A spherical phantom is filled with water solutions containing various amounts of metabolites. It is placed on top of an endo-rectal coil with the balloon filled with per fluorocarbon. MRS data is acquired on a GE 1.5 T MR scanner. The metabolite values, their ratios etc as reported in GE software, FuncTool are studied as functions of metabolite concentrations in the phantom. RESULTS: Analysis of the phantom data indicates that the metabolite ratio reported in FuncTool is approximately linearly correlated to the metabolite concentrations used in the phantom to a certain point and then saturates whereas the largest metabolite value is well correlated with its concentration in the phantom. All metabolite values become weaker and SNR lower as we move away from the coil. CONCLUSIONS: This work indicates the potential of using metabolite values directly provided their spatial dependences on the distance of the voxels from the endo-rectal coil can be accommodated.
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PURPOSE: To commission and verify an Epson scanner for film dosimetry for total skin electron beam therapy (TSEB). METHODS: Use data from an IBA PPC40 parallel-plate ion chamber and Sun Nuclear QED skin diode detectors as standard; we have made comparisons to the film measurement using Kodak XV films. Hurter-Driffield (HD) curve are established for 6 MeV total skin electron beams at a source-to-surface distance (SSD) of 5 m. Also HD curves are built for 6 MeV at a 100 cm SSD. Dose profiles for a series of oblique incident large electron fields are measured using the film for approximately 80 cGy dose delivered at the peak. The film is then scanned using two scanners, an Epson expression 10000 XL and a Vidar VXR-16 Dosimetry Pro. The optimal scanning conditions (e.g., dot per pixel size, internal color correction scheme) are chosen for the Epson scanner. Matlab is then used to analyze the optical density (OD) of the scanned films. A transmission densitometer made by Tobias Associates transmission is used to analyze the films to give a classical standard. RESULTS: The analysis of the Epson scanner is presented in two forms: one with and one without the HD correction from the established HD curve. The error analysis gives an uncertainty of 5% without the HD correction. An improved result of approximately 3% is found when an HD correction is applied to the analysis. CONCLUSIONS: A simple Epson scanner satisfies the commissioning standards for TSEB when an HD curve correction is applied.
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PURPOSE: To describe an efficient method for verifying the size and centering of the radiation field from the Cyberknife IRIS variable collimator with sub-millimeter accuracy using a general purpose commercially available diode array. METHODS: We present a technique using a conventional diode array (Sun Nuclear Profiler) with the array at an extended distance of 320 cm. The projection of the 4 mm diode spacing back to the 80 cm field definition distance gives an effective spacing of 1 mm, sufficient to confirm proper operation of the IRIS. We describe the data acquisition process and present data comparing the Profiler measurements to scanned measurements for both profile and FWHM analysis and reproducibility of the technique over repeated measurements. RESULTS: Average difference between original water scanner measurements and diode array measurements over the 12 aperture sizes) from 5 mm to 60 mm) were - 0.14 mm (range 0.03 mm to 0.83 mm). Reproducibility and centering measurements had a similar range of accuracy. CONCLUSIONS: A general purpose commercially available diode array can be used to quickly and accurately characterize the field size and centering of the Cyberknife IRIS variable collimator system with sub-millimeter accuracy subsequent to service, software recalibration, software upgrades or associated with routine QA. This technique avoids the time consuming and cumbersome water tank scanning with a diode and the difficulties associated with image based measurements (CR or radiochromic film) that require time consuming and careful calibration and choice of threshold values.
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There are many recommendations for appropriate quality control for computed tomography scanners. The breadth of the recommendations is large and this has led to confusion as to what quality control is indeed necessary. The American College of Radiology is producing a QC Manual for CT under the auspices of the CT Accreditation Program. The draft manual is currently under final stages of review. LEARNING OBJECTIVES: 1. Review current recommendations for CT quality control 2. Describe the draft QC program of the ACR CT QC Manual 3. Discuss the status of the ACR CT QC Manual.