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

Abstract

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.