Development of a TOPAS Monte Carlo (MC) model extension to simulate the automatic exposure control function of a C-arm CBCT.

Purpose: Accurate simulation of organ doses in C-arm CBCT is critical for estimating personalised patient dosimetry. However, system complexities such as automatic exposure control (AEC) and the incorporation of DICOM images into simulations are challenging. The aim of this study was to develop a model for mimicking the operation of an AEC system, which maintains a constant dose to the detector through mA modulation in order to facilitate more accurate MC dosimetry models for C-arm CBCT.

Methods: A Siemens Artis Q Interventional Radiology (IR) C-arm system [Siemens, Erlangen, Germany] was modelled in TOol for PArticle Simulation (TOPAS) by incorporating system specifications such as rotational speed, number of projections and exam protocol parameters. A novel threshold scorer, AECScorer, was developed to model the AEC functionality. MC simulations were performed using a variety of imaged volumes including a CTDI phantom, an anthropomorphic phantom and a patient DICOM dataset.

Results: The AECScorer extension provides a framework for a conditional scoring function within TOPAS which allows for the simulation of an AEC system. The AECScorer successfully equalises the dose to the detector for simple phantoms and DICOM imaging datasets by adjusting the number of histories simulated at each CBCT projection. This AECSCorer tool is applicable to other medical imaging systems requiring AEC simulation.

Conclusions: We demonstrate a novel threshold scorer in TOPAS for a C-arm CBCT setup. The presented AECScorer is the first step towards providing a system-, patient- and protocol-specific dose estimates from CBCT dosimetry applications.