HyperSight CBCT dose calculation accuracy for adaptive head and neck radiotherapy: Results from a prospective...

Head and neck (H&N) patients frequently undergo anatomical changes such as weight loss and tumour shrinkage during treatment, requiring repeat imaging and replanning to maintain accurate dose delivery. On-board cone-beam CT (CBCT) systems could support these workflows, however they have historically been limited by reduced Hounsfield unit accuracy, inferior image quality, and susceptibility to metal artifacts from dental hardware, all of which are critical for effective radiotherapy treatment planning and dose delivery. HyperSight CBCT offers improved image quality comparable to FBCT along with metal artifact reduction techniques, but its dosimetric performance in H&N patients, particularly in the presence of dental metal implants, has not yet been evaluated.

This work evaluates the dosimetric accuracy of the HyperSight CBCT system (Varian Medical Systems), including the impact of metal artifact reduction, to determine its reliability for H&N treatment planning.

Images of 25 H&N cancer patients were acquired on a conventional fan-beam CT (FBCT. GE Healthcare) and on two CBCT systems: an Ethos radiotherapy system equipped with HyperSight CBCT, which provides advanced iterative reconstruction (HS-Acuros) and metal artifact reduction reconstruction (HS-MAR) options, and a TrueBeam linear accelerator system with conventional CBCT, which uses filtered back projection reconstruction (Varian Medical Systems). For each patient, the CBCTs were rigidly registered to the FBCT, and the clinically accepted reference plans were forward calculated on all images. Gamma evaluations (3%/3 mm, 2%/2 mm, 1%/1 mm) for all CBCT modes were performed relative to FBCT on slices both unaffected and affected by metal artifacts, with the latter also analyzed as a function of the metal artifact index (AI). DVH metrics were assessed for the planning target volume (PTV), mandible, oral cavity, and parotid glands.

For all gamma criteria, HS-Acuros images demonstrated significantly higher pass rates than both HS-MAR and TrueBeam CBCTs in the artifact-free slices (p ≤ 0.033). In artifact-affected slices, HS-MAR reconstructions performed the best in slices affected by metal artifacts (p ≤ 0.001), exhibiting the least reduction in gamma pass rates with increasing artifact index (AI). Conventional TrueBeam CBCT consistently yielded the lowest gamma pass rates in artifact-prone regions (p ≤ 0.005). Across all slices, the median 3D 3%/3 mm gamma passing rates were above 99.2%, 98.7% and 97.7% for HS-Acuros, HS-MAR and TrueBeam, respectively. Across all CBCT images, the median DVH-metric deviations relative to CTsim FBCT were within 2% for all structures and metrics except for PTV Dmax.

HyperSight provides imaging suitable for direct dose calculation in H&N adaptive radiotherapy with iCBCT MAR improving accuracy in the presence of dental metal artifacts; however, iCBCT Acuros may be preferred when artifact reduction is not critical. This work highlights a significant advancement towards CBCT-based direct-dose calculation for adaptive radiotherapy.