Evaluation of New CBCT Image Reconstruction Algorithms for Proton Adaptive Therapy Workflows
Abstract
Purpose
To evaluate the imaging performance, Hounsfield Unit (HU) accuracy, and proton‑dose calculation implications of three newly released Varian ProBeam 360⁰ CBCT reconstruction algorithms—iCBCT, iCBCT Acuros, and iCBCT MAR Adaptive—compared with the standard filtered back‑projection algorithm, to ultimately characterize image quality, assess metal‑artifact mitigation, and determine suitability for CBCT‑based synthetic CT generation and proton adaptive radiotherapy workflows.
Methods
A Gammex electron‑density phantom and three anthropomorphic phantoms (head, thorax, pelvis) were scanned on the ProBeam 360⁰ CBCT system using four reconstruction algorithms. Each imaging protocol was repeated across anatomical sites using identical acquisition parameters (125 kV, site‑specific mAs). Corresponding reference CT scans were acquired on a Siemens go.Open Pro CT scanner. HU values for all Gammex tissue substitutes and selected anatomical regions were compared between CBCTs and CT. Proton treatment plans were created on the reference CTs for head and thorax phantoms and recalculated on each CBCT dataset to evaluate dose deviations and sensitivity to reconstruction algorithm choice.
Results
For the head imaging protocols, the standard reconstruction algorithm exhibited the largest HU discrepancy between CT and CBCT scans for the electron density inserts, with one outlier found for the general adipose insert. The thorax results were less consistent, with the iCBCT reconstruction algorithms outperforming the standard algorithm for 50% of inserts. Quantitatively assessing the treatment plans generated using anthropomorphic phantoms found the max dose differences (cGy RBE) between the reference and CBCT-forward calculated plan occurred for the standard algorithm, with the lowest dose-differences exhibited by the iCBCT iMAR and iCBCT Acruos algorithms for the head and thorax plans, respectively.
Conclusion
The results suggest the iCBCT-based reconstruction algorithms class exhibits greater fidelity with the reference CT images. Further work is necessary to understand the conditions under which these algorithms are acceptable for proton dose calculation in an adaptive paradigm.