Characterization of the Megavoltage Cone-Beam Computed Tomography (MV-CBCT) System on HalcyonTM for IGRT: Image Quality Benchmark, Clinical Performance, and Organ Doses

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.

Treatment planning comparison of volumetric modulated arc therapy with the trilogy and the Halcyon for bilateral breast cancer

“…Compared with KV imaging, the main disadvantages of MV imaging are higher dose and lower image quality. About Halcyon’s MV CBCT, Malajovich et al [ 19 ] reported that the highest tissue dose of MV CBCT ranges from 2 to 7 cGy per fraction in different treatment sites, which is equivalent to the fractional dose of KV CBCT during breast and pelvic IGRT application, and MV CBCT images of Halcyon is able to identify different soft tissues and lack of metal-induced artifacts. On Halcyon, two different dose image mode could be selected to apply the CBCT.…”

“…On Halcyon, two different dose image mode could be selected to apply the CBCT. Compared to the low-dose mode, the high-quality imaging mode does not provide material advantages [ 19 ]. We found that high-quality CBCT plans increased the OARs doses compared to low-dose CBCT plans.…”

Treatment planning comparison of volumetric modulated arc therapy with Trilogy and Halcyon for bilateral breast cancer

“…Compared with KV imaging, the main disadvantages of MV imaging are higher dose and lower image quality. About Halcyon's MV CBCT, Malajovich et al [19] reported that the highest tissue dose of MV CBCT ranges from 2-7 cGy per fraction in different treatment sites, which is equivalent to the fractional dose of KV CBCT during breast and pelvic IGRT application, and MV CBCT images of Halcyon is able to identify different soft tissues and lack of metal-induced artifacts. On Halcyon, two different dose image mode could be selected to apply the CBCT.…”

Initial Clinical Experience Treating Patients With Gynecologic Cancers on a 6MV Flattening Filter Free O-Ring Linear Accelerator

“…Our study showed that pelvic RT plans, including EFRT plans, for gynecologic cancers on a 6X-FFF ORL were able to meet institutional or RTOG DVH constraints for targets and OARs for all patients, even with 6 (35%) patients receiving an average of approximately 0.04 Gy per fraction of additional dose from MV CBCT. 26 …”