Abstract
Halcyon™ is a single‐energy (6 MV‐FFF), bore‐enclosed linear accelerator. Patient setup is performed by first aligning to external lasers mounted to the front of the bore, and then loading to isocenter through pre‐defined couch shifts. There is no light field, optical distance indicator or front pointer mechanism, so positioning is verified through MV imaging with kV imaging scheduled to become available in the future. TG‐51 reference dosimetry was successfully performed for Halcyon™ in this imaging‐based setup paradigm. The beam quality conversion factor, k Q, was determined by measuring %dd(10)x three ways: (a) using a Farmer chamber with lead filtering, (b) using a Farmer chamber without lead filtering, and (c) using a PinPoint chamber without lead filtering. Values of k Q were determined to be 0.995, 0.996, and 0.996 by each measurement technique, respectively. Halcyon™'s 6 MV‐FFF beam was found to be broader than other FFF beams produced by Varian accelerators, and profile measurements at d max showed the beam to vary less than 0.5% over the dimensions of our Farmer chamber's active volume. Reference dosimetry can be performed for the Halcyon™ accelerator simply, without specialized equipment or lead filtering with minimal dosimetric impact. This simplicity will prove advantageous in clinics with limited resources or physics support.
Highlights
HalcyonTM (Varian Medical Systems, Inc., Palo Alto, CA) is a singleenergy linear accelerator introduced in North America in mid-2017
Because the gantry is enclosed by the bore, patient setup on HalcyonTM is performed by first aligning to external lasers mounted on the front of the bore, and loading to isocenter through pre-defined couch shifts
Beyond requiring tens of minutes to still the water, we found that an over-full water tank may spill onto the couch and gantry cover below, we recommend the water tank not be overfilled and that towels be kept close at hand during couch loading to quickly attend to spills
Summary
HalcyonTM (Varian Medical Systems, Inc., Palo Alto, CA) is a singleenergy linear accelerator introduced in North America in mid-2017. The 6 MV-flattening filter free (FFF) source is mounted opposite an imager and beam-stop stack, and the gantry is enclosed in a carbon fiber bore. Because the gantry is enclosed by the bore, patient setup on HalcyonTM is performed by first aligning to external lasers mounted on the front of the bore, and loading to isocenter through pre-defined couch shifts. Positioning is verified or adjusted using MV imaging: either orthogonal MV image pairs, or MV cone beam computed tomography (CBCT). Performing reference dosimetry for the HalcyonTM differs logistically from other linear accelerators due to its unique features. The American Association of Physicists in Medicine (AAPM) Task Group 51 (TG-51) protocol is a global standard for clinical reference
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