TG-51 reference dosimetry for the Halcyon™: A clinical experience.

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 , was determined by measuring %dd(10) 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 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 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.