Comparing calibration methods of electron beams using plane-parallel chambers with absorbed-dose to water based...

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Radiotherapy Physics

Background:

• Absorbed-dose-based protocols offer two methods for calibrating electron beams with plane-parallel chambers.
• These methods involve direct N(Co)D,w calibration or cross-calibration against a cylindrical chamber.
• Cross-calibration is preferred to circumvent issues with Pwall correction factors at 60Co.

Purpose of the Study:

• To investigate the consistency between direct N(Co)D,w calibration and cross-calibration for PTW Roos, Scanditronics NACP02, and PTW Markus chambers.
• To evaluate the impact of Pwall correction factors and beam quality conversion factors under different dosimetry protocols.
• To assess the validity of fluence perturbation correction factors at d(max) versus d(ref) for well-guarded chambers.

Main Methods:

• Experimental derivation of wall correction factors (Pwall) in 60Co beams and absorbed-dose beam quality conversion factors for 20 MeV electrons.
• Cross-calibration of plane-parallel chambers against a cylindrical ionization chamber.
• Data processing using both the AAPM TG-51 and IAEA TRS-398 dosimetry protocols.

Main Results:

• Systematic differences of up to 1.6% were observed between experimental Pwall values and Monte Carlo calculations for AAPM TG-51.
• Differences of up to 0.6% were found when comparing with the IAEA TRS-398 protocol.
• These Pwall differences directly impact beam quality conversion factors, highlighting the importance of cross-calibration.

Conclusions:

• Cross-calibration is crucial for accurate electron beam dosimetry with plane-parallel chambers due to experimental Pwall data variability.
• For well-guarded chambers, fluence perturbation correction factors at d(max) are comparable to those at d(ref).
• The PTW Markus chamber's factor variations align with established relationships to average energy at depth.