(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To demonstrate that measurements in air or in a single section of the 30 cm diameter, 60 cm long ICRU/AAPM phantom described in AAPM Report 200 may be sufficient for verification of the dose absorption properties of the entire phantom.
Methods: helical scans along the central axis and at the periphery (13.37 cm from isocenter) of the ICRU/AAPM phantom were acquired at multiple tube potentials for several vendors. The instantaneous dose rate dK/dt was recorded using a 0.6 cc thimble ion chamber for each of these scans and processed using the methods of Report 200. The process was repeated on a short (20 cm long) section of the phantom and again in air with the air measurements performed at both radii. The CTDIvol was also measured for reference. Some of the scans were repeated using a standard 10 cm long pencil chamber for comparison. The scans were performed with the smallest collimation and lowest pitch available to maximize the spatial bandwidth.
Results: the Philips Brilliance 64 scanner at 120 kV, Integrated dose for the 20 cm section was 96% of the integral for the central 20 cm of the full scan at the edge and 91% along the central axis. This difference reflects the greatly enhanced relative importance of scatter at the center of the phantom versus the edge. In air, the ratio of the dose at radius 13.37 mm to that at the center increased with kV due to the bowtie filter. These trends occur for all vendors. In air, pencil and thimble chamber measurements were virtually identical.
Conclusion: section measurements provide reliable verification for whole phantom measurements. On and off axis air measurements also appear to be reliable and robust enough for QC verification purposes.
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