Characterization of a prototype rapid kilovoltage x‐ray image guidance system designed for a ring shape radiation therapy unit

Abstract

Purpose This study aims to characterize the performance of a prototype rapid kilovoltage (kV) x‐ray image guidance system onboard the newly released Halcyon 2.0 linear accelerator (Varian Medical Systems, Palo Alto, CA) by use of conventional and innovatively designed testing procedures. Methods Basic imaging system performance tests and radiation dose measurements were performed for all eleven kV‐cone beam computed tomography (CBCT) imaging protocols available on a preclinical Halcyon 2.0 LINAC. Both conventional CBCT reconstruction using the Feldkamp–Davis–Kress (FDK) algorithm and a novel, advanced iterative reconstruction (iCBCT) available on this platform were evaluated. Standard image quality metrics, including slice thickness accuracy, high‐contrast resolution, low‐contrast resolution, regional uniformity and noise, and CT Hounsfield unit (HU) number accuracy and linearity were evaluated using a manufacturer‐supplied QUART phantom (GmbH, Zorneding, Germany) and an independent image quality phantom (Catphan 500, The Phantom Laboratory, New York, NY). Due to the simplified design of the QUART phantom, we developed surrogate and clinically feasible strategies for measuring slice thickness and high‐ and low‐contrast resolution. Imaging dose delivered by these eleven protocols was measured using a computed tomography dose index phantom and pencil chamber with commonly accepted methods and procedures. A subset of measurements were repeated on a conventional C‐arm LINAC (TrueBeam and Trilogy, Varian Medical System) for comparison. Clinical patient images of pelvic and abdominal regions are also presented for qualitative assessment as part of a feasibility study for clinical implementation. Results Image acquisition time was 17–42 s on the Halcyon system compared with 60 s on the C‐arm LINAC systems. The kV imager projection offset, imaging and treatment isocenter coincidence and the couch three‐dimensional match movement all achieved less than1 mm mechanical accuracy. All major image quality metrics were within either the national guideline or vendor‐recommended tolerances. The designed surrogate approach with the QUART phantom showed a range of 0.24–0.35 cycles/mm for spatial resolution, a contrast‐to‐noise ratio (CNR) of 2–20 for FDK reconstruction and a tolerance of 0.5 mm for slice thickness. Other metrics derived from the Catphan images obtained on the Halcyon and C‐arm LINACs showed comparable values for the FDK reconstruction. The iterative reconstruction tended to reduce noise, as evidenced by a higher CNR ratio. The fast scan pelvis protocols for Halcyon resulted in 50% lower dose compared to the standard scans, and the thorax fast protocol similarly delivered 10% lower dose than the standard thoracic scan. Preliminary patient images indicated that rapid kV CBCT with breath‐hold is feasible, with improved imaging quality compared to free‐breathing scans. Conclusion Independent and comprehensive characterization of the kV imaging guidance system on the Halcyon 2.0 system demonstrated acceptable image qual...

Field‐in‐field breast planning for a jawless, double‐stack MLC LINAC using flattening‐filter‐free beams

“…Due to the kVCBCT’s lower imaging dose and fast acquisition time (as low as 16.7 s), we anticipate its wide usage. Image acquisition for Halcyon thus is faster than TrueBeam due to the fourfold increase in gantry rotation speed . In conclusion, treatment delivery times should be comparable between both treatment machines given that patient setup times are significantly longer than beam on times for this type of treatment.…”

“…Maximum dose rate for a flattened 6 MV or 10 MV is 600 MU/min at isocenter for TrueBeam and 800 MU/min at isocenter for the 6FFF beam provided on the Halcyon LINAC. Halcyon was designed to address the global need for access to radiation therapy with integrated imaging guidance, improved clinical efficiency, and shorter installation and commissioning time as well as operational demands . However, the jawless design and unflattened beam provide some challenges when creating breast 3D plans with the FiF techniques.…”

Experience in commissioning the halcyon linac

“…Our test systems, in common with some systems currently available, only offer megavoltage imaging. The Halcyon is designed, however, to facilitate the addition of kV imaging with the imaging beamline perpendicular to the treatment beamline (the same as with most other medical linacs) – at the time of writing, this has just become commercially available and has been characterized by Cai et al Kilovoltage imaging may allow a means to facilitate equipment setup during commissioning – although we found that the MV imaging was sufficient, and may be preferable as it uses the treatment beam line. Results from other commissioning tests are provided in the supplemental material (Data & Appendix ).…”

Clinical safety assessment of the Halcyon system

“…There are a number of publications that either evaluate the operation of the Halcyon or provide recommendations on its clinical use . A recent publication provided a risk assessment of Halcyon acceptance testing and commissioning processes using the TG‐100 methodology .…”

“…There are a number of publications that either evaluate the operation of the Halcyon or provide recommendations on its clinical use. [1][2][3][4][5][6][7][8][9][10][11] A recent publication provided a risk assessment of Halcyon acceptance testing and commissioning processes using the TG-100 methodology. 12 In that work, 142 failure modes were identified with the top risk priority number (RPN) values ranging from 85.5 to 95.0 for the acceptance testing and commissioning, respectively.…”