Program Information
Comparison of Absorbed Dose Based On Air-Kerma Calibration and 60Co Absorbed-Dose Calibration in Diagnostic X-Ray Beams
F Araki*, S Umeno, T Ohno, Kumamoto University, Kumamoto, JAPAN
Presentations
SU-H1-GePD-I-3 (Sunday, July 30, 2017) 3:00 PM - 3:30 PM Room: Imaging ePoster Lounge
Purpose: To compare the absorbed dose to water by new dosimetry with the 60Co absorbed dose-to-water calibration with that based on the air-kerma calibration, in diagnostic x-ray beams.
Methods: Thirty fluence spectra were calculated by a SpecCalc program for aluminum half value-layers (Al-HVLs) of 1.5 to 8 mm with tube voltages of 50 to 120 kVp. Using these spectra, the absorbed dose with the 60Co absorbed-dose to water calibration was obtained using a beam quality conversion factor, kQX, for 60Co to x-ray beams at a 1 cm depth in water for a 20 x 20 cm2 field. kQX for a PTW 30013 Farmer ionization chamber was calculated with EGSnrc/cavity and egs_chamber codes. The absorbed dose to water based on the air-kerma calibration obtained according to the AAPM TG-61 protocol. The Farmer ionization chamber was calibrated with the air-kerma for effective energies of 25 to 55 keV and the 60Co absorbed dose to water. The absorbed dose based on both calibrations was compared at a surface and a 1 cm depth in water.
Results: Effective energies for 1.5 to 8 mm Al-HVLs were 25.7 to 54.3 keV, respectively. kQX for 1.5 to 8 mm Al-HVLs was 0.830 to 0.894, respectively. The surface doses in water based on the air-kerma calibration were 6-8% higher than those of the 60Co calibration. Similarly, the doses at a 1 cm depth in water based on the air-kerma were 8% and 4% higher than those of the 60Co calibration for 3 to 8 mm HVLs, respectively. The dose discrepancy between both calibrations may mostly be attributed to the uncertainty in the absorbed dose with the air-kerma calibration procedure.
Conclusion: The surface doses in water based on the air-kerma calibration were systematically 6-8% higher than those of the 60Co calibration.
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