CBCT Imaging Optimization, Towards Establishment of Dose Reference Levels
Abstract
Purpose
To provide practical guidelines for imaging dose optimization and establishment of dose reference levels for CBCT in radiation therapy. Such guidelines are essential to ensure adherence to ALARA principles without compromising patient positioning accuracy. Although often overlooked, imaging contributes to out-of-field exposure and may have stochastic effects on patients, especially younger ones with longer post therapy survival.
Methods
CBCT imaging optimization studies from two institutions are presented. Imaging protocols were modified by reducing the current–time product (mAs) and/or the number of projection frames. Image quality was evaluated using both qualitative and quantitative assessments, followed by clinical image review from multidisciplinary teams composed of medical physicists, radiation oncologists and radiation therapists. Qualitative analysis examined the visibility of targets and anatomical landmarks, while quantitative metrics included image noise, contrast, visibility, and registration accuracy.
Results
Qualitative image assessments revealed increased noise with mAs reduction, and streaking and aliasing artifacts with frame number reduction. However, these did not necessarily render the lower dose protocols unusable, particularly when the intent of imaging was alignment based on bony or external anatomy. Quantitative analyses showed degradation of certain image quality metrics, such as noise, low contrast resolution, and visibility, with minimal effect on high contrast resolution and geometric fidelity. The impact of dose reduction on image registration accuracy remained within clinically acceptable limits across optimized protocols.
Conclusion
CBCT protocol optimization is necessary where low contrast resolution is not critical, and for smaller patients. Phantom studies can serve as a foundation for evaluating altered protocols, but patient-based validation is essential to develop clinically relevant protocols. Quantitative image quality and registration accuracy metrics can guide optimization and assess protocol performance; however, imaging intent often dictates the balance between dose, image quality and clinical utility. These studies can be incorporated into larger efforts to establish DRLs for CBCT imaging.