Comprehensive Multi-Platform Characterization of an Advanced Linac-Based Cone-Beam CT Imaging System
Abstract
Purpose
Recent advances in cone-beam CT (CBCT) technology, such as large field of view and advanced iterative reconstruction algorithms, offer potential for using these on-board imagers for adaptive radiation therapy (ART). This study quantitatively characterizes the density-Hounsfield unit (HU) relationship across several platforms and quantifies inter-machine variability to establish the feasibility of implementing CBCT-based dose calculation in routine ART and simulation-free treatment planning workflows.
Methods
Five linear accelerators of varying age (1-8 years) and platform (4 TrueBeams and an Edge) with Varian Hypersight were evaluated. CBCT scans of a body and head electron density phantom were performed with varying density rod configurations across different imaging protocols and reconstructions (standard, iterative CBCT, iterative CBCT Fast, and iterative CBCT metal artifact reduction). Scans were reacquired over 4 months to evaluate HU repeatability. HU values were compared across the machines to evaluate inter-machine variability. One machine had a kV tube and panel replacement; scans were reacquired after each major service event to evaluate the impact on the resulting HU.
Results
Variability with density rod placement and across machines was observed. One machine was an outlier from the other machines; following kV tube replacement, the machine was similar to the other machines. The kV panel replacement had less of an impact on the resulting HU. HU versus density curves for the different protocols and machines also show disagreements across the platforms for some protocols. Comparison of repeat scans on the same platform had a maximum difference of 40 HU (2.7%) for the Edge and 19 HU (1%) for a TrueBeam.
Conclusion
For some densities and protocols, the variability with density rod position, in addition to inter-machine variability, exceeds published HU tolerances for accurate dose calculation. This potentially indicates the need for individual machine density to HU tables for dose evaluation and adaptive re-planning.