Measurement-anchored Monte Carlo prediction of diagnostic x-ray air kerma using HVL-matched spectra and per-electron normalization

Abstract

Background

Quantitative Monte Carlo (MC) prediction of diagnostic x-ray air kerma is limited by mismatch between theoretical spectra and measured beam quality and by the lack of a reproducible normalization from source-photon tallies to mAs-based exposures.

Purpose

To test whether HVL-matched spectra combined with measurement-anchored per-electron normalization can predict free-in-air and transmitted air kerma under selected radiographic exposure settings and to evaluate the framework in PHITS and EGS5.

Methods

Birch-Marshall spectra at 50, 80, and 120 kV were adjusted by tuning the Al-equivalent filtration parameter so that the theoretical HVL agreed with the measured HVL. The matched spectra were used as common input for PHITS and EGS5. Free-in-air air kerma was measured for 102 exposure conditions, and transmitted air kerma was measured after 2.0-cm PMMA or 0.5-cm Al for seven conditions at each tube voltage. From measured air kerma and MC air-kerma tallies expressed per source photon, we defined the normalization constant $Y\left(V,I,t\right)$, the per-electron correction factor $X\left(V,I,t\right)$, and the tube-voltage-averaged coefficient $X^{\prime }\left(V\right)$. Predictions based on $X^{\prime }\left(V\right)$ were compared with measurements using percentage error. Performance on the full free-in-air dataset was interpreted as agreement within the calibration dataset, whereas held-out-condition performance was assessed by leave-one-out cross-validation (LOOCV) on seven selected radiographic exposure settings at each tube voltage. Combined standard uncertainty was also summarized.

Results

After HVL matching, the free-in-air MC air-kerma tallies expressed per source photon differed between PHITS and EGS5, a finding interpreted primarily in relation to differences in source definition and source normalization. After measurement-based normalization with $X^{\prime }\left(V\right)$, predicted air kerma was generally consistent with measurements under both free-in-air and transmitted conditions. The largest mean percentage error was 7.8%, slightly exceeding the predefined ± 7.5% operational benchmark. The representative combined standard uncertainty was 5.6%; when the nominal tube-voltage setting was used directly as a simulation input, inclusion of the tube-voltage accuracy term increased it to 7.3%.

Conclusions

Within the tested radiographic system and slab-transmission geometries, HVL-matched spectra combined with measurement-anchored per-electron normalization enabled prediction of air kerma under actual exposure conditions. Despite differences in the MC air-kerma tallies expressed per source photon, the final predicted values were generally consistent with measurements in both codes under the tested conditions.