Personalised Dosimetry for Cone Beam Computed Tomography (CBCT); Monte Carlo and Empirical Approaches

Cone beam computed tomography (CBCT) is a modern imaging technique in radiology which produces a divergent cone-shaped source of ionising radiation using a fixed X-ray source and detector on a rotating gantry. While the advancements in CBCT technology have been rapid, the techniques for accurately calculating the CBCT contribution to patient dose have not kept pace in diagnostic radiology. Patient and system specific dosimetry in diagnostic CBCT is an area of active research, with several studies recognising the necessity of assessing individual patient organ doses. The wide range of equipment, complex system configurations and exposure parameters used in CBCT presets challenges for estimating patient organ doses. As a result, the current clinical implementation of personal dosimetry models that account for individual patient-specific factors, such as body size, anatomical variations and exam exposure parameters, remain limited. This research aimed to develop a novel MC approach to CBCT dosimetry, offering more tailored patient dose estimates by incorporating system complexities, CBCT rotational geometries, exam specific parameters and patient imaging data. To achieve this, standard metrics for quantifying image quality (IQ) were used to characterise two commonly used clinical abdominal CBCT protocols of two C-arm CBCT systems, a Siemens Artis Q and a Philips Azurion M20. Initial validation of the MC models involved the quantification of MC estimates and validation with empirical measurements in simple geometrical phantom studies. A novel conditional scoring algorithm, the AECScorer, was developed with the aim of addressing the lack of AEC representation in the Siemens system MC simulation. Subsequent MC simulations were carried out on a more complex geometry, an anthropomorphic phantom, and the results were compared with those determined experimentally using a CIRS® anthropomorphic phantom and thermoluminescent dosimeters (TLDs). Finally, a preliminary patient dosimetry investigation using the developed, and validated, MC model for a small cohort of patient images was carried out. By incorporating system-specific parameters from two C-arm CBCT systems, it enabled the calculation of abdominal absorbed doses for individual patients using CBCT images. Both systems offered a method to account for the AEC, with results in good agreement with published CBCT organ dosimetry. Although associated limitations were identified, this component of the study provided a novel, proof-of-concept demonstration of the use of patient images in MC simulations towards robust, scanner specific, protocol specific personalised patient dosimetry in CBCT.