Characterization of secondary-radiation background in X-ray flat-panel detectors during scanning proton beam irradiation.

BACKGROUND

Secondary-radiation background (BG) generated in kilovoltage X-ray flat-panel detectors (FPDs) under scanning proton beam irradiation has not been thoroughly analyzed.

PURPOSE: This study aimed to determine and mathematically model the characteristics of secondary-radiation BG in FPDs under scanning proton beam irradiation.

METHODS

Using a synchrotron-based proton system and two FPDs mounted on a gantry tilted at 135° to the proton beam axis, we acquired images of two phantoms (block and thorax). The FPD images were captured during three conditions: only X-ray exposure, proton irradiation, and pauses between proton irradiations. Because the FPD readout rate (30 fps) was twice the X-ray exposure rate (15 pps), the images were further categorized into two types: those corresponding to with and without X-ray exposure. These FPD images were analyzed to determine the characteristics of the secondary-radiation BG. In addition, a Monte Carlo simulation was conducted to complement the performed measurements.

RESULTS

Analysis of the FPD images without X-ray exposure revealed that the pure secondary-radiation BG appeared as sparse spike patterns without a noticeable spatial bias across the FPD area in most images. The BG affected only 1.25% of the FPD pixels and could be modeled as an exponentially decreasing function with increasing pixel intensity. When considering typical X-ray image brightness for the block and thorax phantoms, the impact of the BG on mean-pixel-intensity variation was < 1%. Monte Carlo simulations suggested that prompt photons were the primary source reaching the FPD.

CONCLUSIONS

This study demonstrates that the secondary-radiation BG characteristics observed in the X-ray FPD images during scanning proton beam irradiation can be quantitatively modeled. The impact of the secondary-radiation BG on mean-pixel-intensity of FPD images was minimal.