Quantification of beam steering with an ionization chamber array.

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Routine quality assurance for linear accelerators (linacs) typically relies on water scanning systems for beam steering verification.
• There is a need for efficient and accurate methods to ensure beam symmetry in radiation therapy delivery.

Purpose of the Study:

• To establish and validate a beam steering procedure utilizing a 2D ionization chamber array (ICA).
• To verify the equivalence of beam symmetry measurements between the ICA and a standard water scanning system (3D Scanner, 3DS).
• To evaluate the calibration accuracy, reproducibility, and stability of the ICA for beam steering applications.

Main Methods:

• Forty-five photon and 80 electron beams from Varian C-series and TrueBeam linacs were steered using a calibrated 2D ICA.
• Beam profiles were re-measured post-steering with both the ICA and a 3D water scanning system for comparison.
• Symmetry was quantitatively compared using point-by-point differences, histogram analysis, and adherence to Varian's protocol.

Main Results:

• ICA array calibrations demonstrated high reproducibility (within 0.5%) and stability over several years.
• Uncertainty in beam symmetry measurement due to ICA calibration was less than 0.5%.
• Post-steering, 95% of symmetry comparisons between ICA and 3DS agreed within 0.7%, with 100% agreement within 1.0%.
• Steering with the ICA resulted in 97.8% of photon and 97.5% of electron beams exhibiting symmetry within 1% when measured by 3DS.

Conclusions:

• A calibrated 2D ionization chamber array (ICA) provides accurate and reproducible beam steering for linear accelerators.
• The ICA method is equivalent to traditional water scanning systems for verifying beam symmetry.
• This validated ICA procedure enables efficient quality assurance, ensuring precise beam symmetry crucial for radiation therapy.