Cylindrical ionization chamber response in static and dynamic 6 and 15 MV photon beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dose measurement is critical in radiation therapy.
• Ionization chambers are standard tools for dose verification.
• Non-reference beam conditions present challenges for accurate dosimetry.

Purpose of the Study:

• To assess the performance of the CC13 ionization chamber under non-reference photon beam conditions.
• To investigate chamber response in penumbra and build-up regions for static and dynamic intensity-modulated beams.
• To quantify variations in chamber response and determine necessary correction factors.

Main Methods:

• Performed measurements with a CC13 ionization chamber in static 6 MV photon fields (100x100, 20x100, 20x20 mm²).
• Utilized Monte Carlo simulations (EGSnrc egs_chamber) for static fields and dynamic 6/15 MV beams with a Varian multi-leaf collimator.
• Calculated conversion factors, correction factors, and point-dose correction factors to analyze chamber response variations.

Main Results:

• Correction factors were generally close to unity (0.98-1.02) on the central axis and penumbra center for static fields.
• Largest corrections were needed off-axis beyond the penumbra center.
• Point-dose correction factors varied (0.54-0.71) and were influenced by field size and depth.
• Chamber response in dynamic fields showed similar trends to static fields, with factors of 0.99-1.03 inside/near the penumbra and 0.92-0.94 beyond.

Conclusions:

• The CC13 chamber requires corrections for accurate profile measurements in narrow (20 mm) static fields.
• Measurements in dynamic small fields, simulating pre-treatment quality assurance, were feasible.
• Uncorrected chamber response may be acceptable for dose verification in high-gradient dynamic fields within a 3% tolerance.