(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: investigate a relationship between backscatter factors (BSFs) and Al half-value layer (Al-HVL) by using a parameter of quality index (QI) for the diagnostic kilovoltage x-ray beams.
Methods: BSFs for x-ray spectra with different tube voltages were obtained from the weighted average BSFs, which were calculated by convolution of BSFs for monoenergetic photons between 8 and 140 keV at field sizes from 10 × 10 to 40 × 40 cm². The BSFs for each monoenergetic photon were calculated from the ratio of dose at the surface of a water phantom to that at the same point in free air using the EGSnrc/cavity code. The weighted average BSFs were verified by comparing them with directly calculated BSFs for x-ray spectra. The x-ray spectra were generated with a SpekCalc program, for Al-HVL values of 0.5-13.2 mm and tube voltages of 40-140 kVp. In the MC simulations, the photon and electron cutoff energies were set to 1 keV and 516 keV, respectively. The statistical uncertainty (k = 1) of the calculated doses was less than 0.1%. Finally, the weighted average BSFs for x-ray spectra were expressed as a function of Al-HVL by using a parameter of QI of 0.35, 4, 5, 6, and 0.7, which are specified by the ratio of effective photon energy and tube voltage (kVp).
Results: weighted average BSFs were in good agreement of less than 0.8% with directly calculated values from the x-ray spectra. Finally, the BSFs were a good correlation with Al-HVL by using a parameter of QI of 0.35-0.7 for the x-ray spectra with different tube voltages, compared to the existing simple relationship of BSFs and Al-HVL.
Conclusion: The BSFs were a good correlation with Al-HVL by using a parameter of QI and it is useful for diagnostic kilovoltage x-ray beam dosimetry.
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