Determination of backscatter factors based on the quality index for diagnostic kilovoltage x-ray beams.

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, B-w, for x-ray fluence spectra were calculated from the weighted average of B-w for monoenergetic photons of between 8 and 140 keV with field sizes of 10 cm x 10 cm to 40 cm x 40 cm. The value of B-w 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 B-w 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 B-w 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 R-2 > 0.99 with Al-HVL for the given QI values. Conclusions: It is possible to obtain B-w more accurately by using QI specified by the measured Al-HVL.