A Monte Carlo study of organ and effective doses of cone beam computed tomography (CBCT) scans in radiotherapy.

Cone-beam CT (CBCT) scans utilised for image guided radiation therapy (IGRT) procedures have become an essential part of radiotherapy. The aim of this study was to assess organ and effective doses resulting from new CBCT scan protocols (head, thorax, and pelvis) released with a software upgrade of the kV on-board-imager (OBI) system. Organ and effective doses for protocols of the new software (V2.5) and a previous version (V1.6) were assessed using Monte Carlo (MC) simulations for the International Commission on Radiological Protection (ICRP) adult male and female reference computational phantoms. The number of projections and the mAs values were increased and the size of the scan field was extended in the new protocols. Influence of these changes on organ and effective doses of the scans was investigated. The OBI system was modelled in EGSnrc/BEAMnrc, and organ doses were estimated using EGSnrc/DOSXYZnrc. The MC model was benchmarked against experimental measurements. Organ doses resulting from the V2.5 protocols were higher than those of V1.6 for organs that were partially or fully inside the scans fields, and increased by (3-13)%, (10-77)%, and (13-21)% for the head, thorax, and pelvis protocols for both phantoms, respectively. As a result, effective doses rose by 14%, 17%, and 16% for the male phantom, and 13%, 18%, and 17% for the female phantom for the three scan protocols, respectively. The scan field extension for the V2.5 protocols contributed significantly in the dose increases, especially for organs that were partially irradiated such as the thyroid in head and thorax scans and colon in the pelvic scan. The contribution of the mAs values and projection numbers was minimal in the dose increases, up to 2.5%. The field size extension plays a major role in improving the treatment output by including more markers in the field of view to match between CBCT and CT images and hence setting up the patient precisely. Therefore, a trade-off between the risk and benefits of CBCT scans should be considered, and the dose increases should be monitored. Several recommendations have been made for optimisation of the patient dose involved for IGRT procedures.