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Dosimetry for Cell Irradiation using Orthovoltage 40-300 kV X-Ray Facilities
Published on: February 20, 2021
Determination of backscatter factors based on the quality index for diagnostic kilovoltage x-ray beams
1Graduate School of Health Sciences, Kumamoto University, 4-24-1 Kuhonji, Chuo-ku, Kumamoto 862-0976, Japan.
Purpose:
This study aims to investigate the relationship between backscatter factors and Al-half-value-layers (Al-HVL) by making the quality index (QI) a parameter for diagnostic kilovoltage x-ray beams.
Methods:
Backscatter factors, Bw, for x-ray fluence spectra were calculated from the weighted average of Bw for monoenergetic photons of between 8 and 140 keV with field sizes of 10 cm × 10 cm to 40 cm × 40 cm. The value of Bw for monoenergetic photons was calculated from the ratio of the water kerma at the surface of a water phantom and that at the same point free-in-air using the EGSnrc/cavity code. The weighted averaged backscatter factors were validated by comparing them with those of direct Monte Carlo calculations for the x-ray fluence spectra. The Bw for the x-ray fluence spectra were classified by a QI of 0.35, 0.4, 0.5, 0.6, and 0.7 specified by the ratio of the effective energy and maximum energy. The relationship between Bw and Al-HVL was evaluated for the given QI values. The x-ray fluence spectra were generated for tube voltages of 40-140 kVp with Al-HVLs of 0.5-13.2 mm using the SpekCalc program.
Results:
The weighted averaged backscatter factors for x-ray fluence spectra agreed within 0.7% with those of the direct Monte Carlo calculations. The backscatter factors were represented by the fitting curves of R2 > 0.99 with Al-HVL for the given QI values.
Conclusions:
It is possible to obtain Bw more accurately by using QI specified by the measured Al-HVL.
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