Feasibility of HyperSight CBCT for adaptive radiation therapy: A phantom benchmark study of dose calculation accuracy...

The development of on-board cone-beam computed tomography (CBCT) has led to improved target localization and evaluation of patient anatomical change throughout the course of radiation therapy. HyperSight, a newly developed on-board CBCT platform by Varian, has been shown to improve image quality and HU fidelity relative to conventional CBCT. The purpose of this study is to benchmark the dose calculation accuracy of Varian's HyperSight cone-beam computed tomography (CBCT) on the Halcyon platform relative to fan-beam CT-based dose calculations and to perform end-to-end testing of HyperSight CBCT-only based treatment planning.

A HU to mass density curve was measured for the HyperSight CBCT system and implemented into the Eclipse treatment planning system. Following this, computational dosimetric analysis was performed between dose distributions calculated on CT simulation (CT_sim) and HyperSight CBCT images on two anthropomorphic phantoms for pelvic and head and neck treatment sites. Additionally, an end-to-end test was carried out for a head and neck intensity modulated radiation therapy (IMRT) plan.

The HU to mass density curves acquired on CT_sim and HyperSight CBCT were similar (< 30 HU) for near-water equivalent materials, but deviated for high-density materials, with a maximum difference of 150 HU. For dose calculations, excellent agreement between dose calculations performed on CT_sim and HyperSight CBCT phantom images was observed, where three-dimensional gamma pass rates between the two dose distributions were observed to be $\ge$ 90% at 1%/1 mm (5% threshold). For the end-to-end test, absolute doses were verified to within 1% of ionization chamber measurements, while Delta4+ and portal dosimetry measurements yielded passing results (gamma pass rate $\ge$ 90%) down to 2%/2 mm criterion.

In this study, the accuracy of dose calculations performed on HyperSight CBCT was found to be within 1% of CT_sim calculations for pelvic and head and neck treatment sites. End-to-end results using the RANDO anthropomorphic phantom indicate that HyperSight CBCT images are suitable for radiation treatment planning.