TG-21 versus TG-25: a comparison for electrons

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• The American Association of Physicists in Medicine Task Group 21 (TG-21) protocol has guided dosimetry reviews since 1984.
• The subsequent Task Group 25 (TG-25) report supplements TG-21 for electron-beam dosimetry beyond dmax.
• Confusion exists regarding which protocol to use for absolute electron beam calibration.

Purpose of the Study:

• To compare dosimetry discrepancies between TG-21, a modified TG-21 (RPC version), and TG-25.
• To analyze these discrepancies based on electron energy, chamber type, and phantom material.

Main Methods:

• Calculated dose differences using TG-21, RPC-modified TG-21, and TG-25 protocols.
• Evaluated results for 6 and 20 MeV electron beams.
• Assessed variations across cylindrical and parallel plate chambers using water, polystyrene, and acrylic phantoms.

Main Results:

• Largest discrepancies were observed between TG-25 and TG-21 methods for low-energy electrons in water and polystyrene.
• The mean difference across all conditions was 0.8%, with a maximum of 3.3% in polystyrene.
• Key contributors to differences include effective point of measurement, energy determination, stopping power ratios, and correction factors.

Conclusions:

• Discrepancies exist between TG-25 and TG-21 dosimetry protocols, necessitating careful consideration for electron beam calibration.
• Protocol selection impacts absorbed dose determination, particularly for low-energy electrons and specific phantom materials.
• Clarification is needed to ensure consistent and accurate dosimetry in radiation therapy clinical trials.