Comparison of extended kV-CBCT image quality between conventional and O-ring linear accelerators

In cases of non-central or large tumours, the use of an extended field of view is important for patient positioning or adaptive treatment. The aim of our study was to investigate the longitudinal and lateral consistency of extended kilovoltage cone-beam CT (kV-CBCT) image quality over the entire scan length.

The study compared extended kV-CBCT image quality between conventional and O-ring linear accelerators (LINACs) using image quality phantoms. The impact of lateral and longitudinal phantom offsets was analysed across the reconstruction algorithms [filtered back projection - Feldkamp-Davis-Kress (FDK) and iterative cone-beam computed tomography (iCBCT)] and imaging protocols.

The CT calibration curves showed consistent alignment across LINACs, imaging protocols, longitudinal displacements, and reconstruction algorithms. The average mean difference in the CT numbers measured during longitudinal shifts, averaged over the other material inserts, was 37 ± 15 Hounsfield units (HU). In the homogeneity examination, the lateral inserts displayed signal reductions as they moved away from the isocentre. For an imaging protocol on a ring accelerator, the central insert remained near 8 HU, while the signal of lateral inserts located 20 cm from the isocentre decreased from 0 HU to -160 HU.

Our results show that the longitudinal extension does not affect image linearity, making extended CBCT protocols suitable for long target volumes. CBCT-based adaptation may be unsuitable for large lateral or peripheral tumours due to poor image quality. Consequently, the generation of synthetic CTs from peripheral regions of CBCT images should be approached with caution.