Energy dependence of Farmer-type ionization chambers used in kilovoltage x-ray beam reference dosimetry

Absorbed dose determination for therapeutic kilovoltage (kV) x rays is performed following international recommendations to ensure consistency in practice and equivalence in dose delivery for patients worldwide. These recommendations include limits on the variation of ionization chamber calibration coefficients with beam quality. To date, data on energy-dependence have only been published for a limited set of ionization chambers and kV beams, and there are limited guidelines regarding which models of cylindrical ionization chambers are suitable for kV reference dosimetry.

To provide cylindrical ionization chamber energy-dependence data with enough beam qualities and chambers to assist medical physicists to assess the compliance of chamber models with protocol recommendations regarding chamber characteristics.

Air kerma calibration coefficients for 150 cylindrical ionization chamber calibrations performed between 2017 and 2024 in 59 medium-energy x-ray beam qualities at the Australian Primary Standards Laboratory were analyzed. Chamber response was scrutinized and assessed for compliance with international recommendations on absorbed dose determination for kilovoltage x-ray beams.

All chambers showed some degree of energy dependence, with none of the chamber models investigated fully satisfying international recommendations. Chamber response is dominated by filtration (half-value layer), with a weaker dependence on generating potential (kVp); differences of up to ±4.3% were observed for a 100 kVp beam spanning 1.30-6.61 mm Al HVL. Distinct differences in response were observed between chamber models, providing rationale for use or exclusion in kV absorbed dose determination. Energy dependence was observed to vary between chambers of the same model type, indicating that calibration coefficients cannot be interchanged between individual chambers of the same model.

Cylindrical ionization chambers frequently used for reference dosimetry of kV x-ray beams exhibit different energy dependence based on model type. Some models frequently employed and recommended for use in absorbed dose determination do not satisfy international protocol recommendations regarding energy dependence. International recommendations and protocols for absorbed dose determination for kilovoltage x-rays based solely on kVp should be reviewed and further developed to explicitly account for the effect of filtration (HVL) on chamber response. The results of this work support the adoption or refinement of the recommended chamber types within reference dosimetry protocols.