Abstract
Purpose: The Varian Halcyon includes an ultrafast 6 MV flattening filter free (FFF) cone-beam computed tomography (MV-CBCT). Although a kV-CBCT add-on is available, in the basic configuration MV is used for image guided radiotherapy (IGRT). We characterized the MV-CBCT imager in terms of reproducibility, linearity, field of view (FOV) dependence, detectability of soft-tissue, and the effect of metal implants. The performance of the MV-CBCT in the clinic, including resulting dose to organs, is also discussed herein.Methods: A Gammex phantom was scanned using a Halcyon MV-CBCT and a 120 kVp Siemens Definition Edge CT. Mean and standard deviation of Hounsfield Units (HUs) for different electron density relative to water () inserts were extracted. Doses to clinical patients due to MV-CBCT are calculated within Eclipse during treatment planning.Results: A stable and near-linear HU-to- curve was obtained using the MV-CBCT. As the scan length increased from 10 to 28cm, the linearity of curve improved while the mean HUs decreased by 30%. All soft tissue inserts in the Gammex phantom were distinguishable. A crescent artifact affected HU measurements by up to 40 HUs. Soft-tissue contrast was sufficient for clinical online image-guidance in the low dose (5 MU) mode. Mean doses per fraction to organs-at-risk (OARs) were as high as 6 cGy for head and neck, 5 cGy for breast, and 4 cGy for pelvis patients. Metal rods did not affect HU values or introduce noticeable artifacts.Conclusions: Halcyon's MV-CBCT has sufficient soft tissue contrast for IGRT and lacks metal-induced artifacts. Even though the absolute HU values vary with phantom size and scanning length, the HU-to- conversions are linear and stable day-to-day. In clinical cases, highest tissue doses from MV-CBCT ranged from 2-7cGy per fraction for various treatment sites, which could be significant for some organs at risk. Dose to out-of-treatment-field organs can be limited by reducing the scan length definition during planning and using the low dose mode. The high quality imaging mode did not provide material advantages over the low dose mode. Adequate IGRT was successfully delivered to multiple tumor sites using MV-CBCT.
Highlights
The Halcyon system was designed as a vehicle for highthroughput image guided radiotherapy
The absolute Hounsfield Unit (HU) values varies as scatter condition changes, e.g., phantom size and/or scanning length, HU-to-ρWe conversions were linear and stable day-to-day, but still showed scan length dependence and presence of a crescent artifact which should be considered if CBCT images are used for dose calculation
Highest tissue doses from MV-CBCT ranged from 2-7 cGy per fraction for various treatment sites, which is comparable to per fraction dose from kV CBCTs during breast and pelvic IGRT applications, but substantially higher than low dose head and neck protocol
Summary
The Halcyon system was designed as a vehicle for highthroughput image guided radiotherapy. This goal is achieved with a single energy linear accelerator (6 MV FFF) in an enclosed gantry, resulting in faster installation and commissioning, reduced cost of ownership, and streamlined workflows. The clinical beam is available for portal imaging and CBCT. The current Halcyon 2.0 version has an optional on-board kV-CBCT imager which has resulted in images comparable to diagnostic CTs [1]. Clinics looking to install a Halcyon may opt to forego the kV capabilities when the objective is to simplify and lower the cost of radiotherapy, or due to temporary unavailability to users in some regions. One of the main objectives of this study was to investigate the use of MVCBCT in routine treatment using Halcyon’s implementation
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