High-energy-photon dose measurements using exposure-calibrated ionization chambers

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Radiotherapy Physics

Background:

• Accurate absorbed dose determination is crucial for effective radiotherapy.
• Current dosimetry protocols rely on Cobalt-60 exposure-calibrated ionization chambers and beam quality factors (Clambda).
• Uncertainties exist regarding the appropriate use of buildup caps and the accuracy of recommended Clambda values for high-energy photons.

Purpose of the Study:

• To evaluate the impact of buildup caps on the Farmer chamber response in water for high-energy photon beams.
• To assess the accuracy of current Clambda values and their influence on absorbed dose calculations.
• To identify potential sources of error in high-energy photon dosimetry.

Main Methods:

• Farmer chamber response measurements were conducted in water, with and without the Cobalt-60 buildup cap.
• The effect of the buildup cap on Clambda was analyzed at various photon energies (Cobalt-60, 4 MV, and above 20 MV).
• Influence of chamber wall materials and displacement factors on Clambda was theoretically considered.

Main Results:

• The buildup cap showed no significant effect on Clambda at Cobalt-60 and 4 MV.
• An increase of 1%-2% in Clambda was observed with the buildup cap for photon energies above 20 MV.
• Potential errors of 3%-5% in absorbed dose determination were identified at high energies due to current Clambda values and chamber characteristics.

Conclusions:

• The use of buildup caps can introduce small but measurable changes in Clambda at high photon energies.
• Current Clambda values may lead to significant errors in absorbed dose calculations for high-energy photon beams.
• Chamber design (wall materials, cap, cavity volume) and measurement conditions critically affect dosimetry accuracy.