A comparison between calculated and experimental kQ photon beam quality correction factors

Area of Science:

• Medical Physics
• Radiotherapy Dosimetry
• Radiation Oncology

Background:

• The International Code of Practice provides calculated kQ factors for high-energy photon beams.
• Accurate dosimetry is crucial for effective radiotherapy treatment.
• Validation of calculated dosimetry factors against experimental data is essential.

Purpose of the Study:

• To validate calculated kQ factors for high-energy photon beams against experimental data.
• To assess the accuracy of absorbed dose to water determinations using calculated kQ factors.
• To evaluate the suitability of TPR(20,10) as a photon beam quality specifier.

Main Methods:

• Compilation of experimental kQ values for various ionization chambers (NE2561/2611, NE2571, PTW30001, PR06).
• Inclusion of energy dependence of G(Fe3+) ratio for Fricke-derived values.
• Comparison of calculated kQ values with experimental data across a range of photon beam qualities.

Main Results:

• Calculated kQ values are a very good estimate for three of the four chamber types analyzed.
• For the NE2571 chamber, a small energy-dependent difference (max 0.4%) between calculated and experimental kQ was observed.
• Good agreement was found using TPR(20,10) as the beam quality specifier for most data, with exceptions at Canadian NRC.

Conclusions:

• Calculated kQ factors provide accurate absorbed dose to water determinations when individual chamber calibration is not feasible.
• The use of calculated kQ factors is justified within the uncertainty limits of experimental data.
• TPR(20,10) remains a suitable photon beam quality specifier despite minor discrepancies with specific high-energy soft beams.