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

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Equipment

Background:

• The Halcyon™ is a novel single-energy (6 MV-FFF), bore-enclosed linear accelerator.
• Patient positioning relies on external lasers and pre-defined couch shifts, lacking traditional light field or optical distance indicators.
• Position verification is currently performed using MV imaging, with kV imaging planned for future implementation.

Purpose of the Study:

• To perform TG-51 reference dosimetry for the Halcyon™ accelerator within its unique imaging-based setup paradigm.
• To determine the beam quality conversion factor (kQ) using various measurement techniques.
• To assess the dosimetric characteristics and simplicity of reference dosimetry for this new accelerator system.

Main Methods:

• TG-51 reference dosimetry protocols were applied to the Halcyon™ 6 MV-FFF beam.
• The beam quality conversion factor (kQ) was measured using a Farmer chamber with and without lead filtering, and a PinPoint chamber without lead filtering, by analyzing %dd(10)x.
• Beam profiles at dmax were measured to evaluate beam characteristics.

Main Results:

• Successful TG-51 reference dosimetry was achieved for the Halcyon™.
• Measured kQ values were 0.995, 0.996, and 0.996 for the different measurement techniques.
• The Halcyon™ 6 MV-FFF beam exhibited a broader profile than other Varian FFF beams, with less than 0.5% variation across the Farmer chamber's active volume.

Conclusions:

• Reference dosimetry for the Halcyon™ accelerator can be performed simply and without specialized equipment or lead filtering.
• The dosimetric impact of simplified methods is minimal.
• This ease of use is a significant advantage for clinics with limited resources or physics support.