VMAT and IMRT plan-specific correction factors for linac-based ionization chamber dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Current radiotherapy calibration protocols do not fully account for patient-specific delivery modulations.
• Detector response in complex clinical fields requires further investigation.
• Accurate absorbed dose to water determination is crucial for effective radiotherapy.

Purpose of the Study:

• To determine correction factors for the Exradin A1SL scanning chamber in a wide range of composite clinical fields.
• To identify chamber-specific perturbations significantly impacting correction factors.
• To assess the suitability of the Exradin A1SL chamber for absolute dosimetry in non-standard radiotherapy fields.

Main Methods:

• Utilized EGSnrc Monte Carlo simulations with a validated BEAMnrc model for 131 patient deliveries (834 beams).
• Embedded the Exradin A1SL scanning chamber in a water-equivalent phantom for simulations.
• Investigated chamber perturbation factors to determine plan-specific correction factors.

Main Results:

• The Exradin A1SL scanning chamber is largely suitable for absolute dosimetry in high-dose, low-gradient regions of composite fields.
• Volume averaging and replacement perturbations were the most significant contributors to correction factors.
• While most individual beams and plans required minimal corrections (<2%), a notable percentage of VMAT beams and plans did require larger corrections.

Conclusions:

• The Exradin A1SL scanning chamber is appropriate for absolute dosimetry in specific regions of intensity-modulated radiotherapy (IMRT) plans.
• Correction-free absorbed dose-to-water measurements are not guaranteed, even with large composite dose distributions relative to detector size.
• Further investigation into plan-specific corrections is necessary for optimal accuracy.