Reference dosimetry condition and beam quality correction factor for CyberKnife beam

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• CyberKnife beams present unique challenges for standard reference dosimetry protocols due to their treatment head and collimating system.
• Current reference dosimetry for CyberKnife is non-uniform, often using a temporary beam quality correction factor (kQ) from 6 MV linear accelerators.
• This necessitates new reference conditions and kQ specific to CyberKnife beams.

Purpose of the Study:

• To improve the certainty of absorbed dose determination for reference dosimetry in CyberKnife beams.
• To establish appropriate reference conditions and determine the beam quality correction factor (kQ) for CyberKnife.
• To provide accurate dosimetry for precise radiation therapy delivery.

Main Methods:

• Analyzed dose flatness and dosimeter reading errors considering radiation fields and detector size to define reference conditions.
• Simulated photon and electron energy spectra in water using BEAMnrc for CyberKnife beams.
• Determined and tabulated beam quality correction factors (kQ) for ionization chambers as a function of beam quality index.

Main Results:

• CyberKnife beams exhibit inferior dose flatness compared to standard 6 MV linear accelerators due to the absence of a flattening filter.
• Ionization chamber cavity length significantly impacts absorbed dose measurement accuracy; a 1.0 cm or shorter cavity is recommended.
• The photon energy spectrum of CyberKnife beams is softer than that of standard 6 MV linear accelerators.

Conclusions:

• Recommended maximum field size with a 6.0 cm collimator and ionization chambers with 1.0 cm or shorter cavity length for CyberKnife reference dosimetry.
• Successfully determined and tabulated beam quality correction factors (kQ) for CyberKnife beams, improving dose accuracy.
• The findings provide a foundation for standardized and accurate dosimetry in CyberKnife radiation therapy.