Simultaneous Optimization of Radiation-Imaging Coincidence for a Multi-Energy Linac

Area of Science:

• Medical Physics
• Radiation Oncology
• Medical Imaging

Background:

• Medical physics guidelines emphasize radiation-imaging coincidence for stereotactic treatments.
• Multi-energy linear accelerators often have a single imaging isocenter, posing a challenge.
• Optimizing isocenter coincidence across all photon energies is critical for treatment accuracy.

Purpose of the Study:

• To develop and validate a procedure for simultaneous optimization of radiation-imaging isocenter coincidence.
• To ensure accurate alignment for all photon energies on an Elekta Versa HD linear accelerator.
• To minimize radiation-imaging isocentricity for enhanced treatment precision.

Main Methods:

• A novel method combining ion chamber measurements and a modified Winston-Lutz (WL) test was used.
• Beam centers were adjusted to the collimator rotation axis at specific gantry, couch, and collimator angles.
• Multi-field WL tests were performed to determine and calibrate the average linac and couch isocenter positions.

Main Results:

• Beam centers were calibrated within 0.10 mm of the collimator rotation axis.
• Linac isocenter coincidence was achieved within 0.20 mm for all energies.
• Average radiation-imaging isocentricity was 0.89 mm, with couch isocenter coincidence within 0.20 mm.

Conclusions:

• A practical method for adjusting radiation-imaging coincidence within 1.0 mm for all energies on the Versa HD was established.
• This procedure enhances the precision of stereotactic treatments by improving isocenter alignment.
• The findings are significant for medical physicists and radiation oncologists utilizing multi-energy linear accelerators.