Effective point of measurement calculations for cylindrical ionization chambers in kilovoltage x-ray beams

Objective: Knowledge of the effective point of measurement (EPOM) of ionization chambers is needed in relative dosimetry for kilovoltage x-ray beams. This study used Monte Carlo calculations to determine EPOM of three cylindrical ionization chambers in kilovoltage x-ray beams. Approach: The EGSnrc Monte Carlo code was used for all calculations for central axis depth doses in a water phantom and in the air cavity of cylindrical ionization chambers in water. EPOM shifts were calculated for x-ray beams from 50 kVp to 300 kVp with HVL’s from 0.07 to 3.5 mm Cu, with field sizes from 5 to 15 cm diameter. Three cylindrical ionization chambers (NE 2571, Exradin A12 and PTW 30013) were realistically modelled in EGSnrc. The EPOM shift value was identified from the minimum deviation in the ratio of the dose-to-water and the dose-to-air in the chamber cavity, following published numerical methods. Main Results: EPOM shifts normalised to the radius (r) of the cylindrical chamber of the air cavity were calculated to be within 0.2r of the central axis for HVLs between 0.5 and 3.5 mm Cu. For HVLs less than 0.5 mm Cu, the EPOM shift increased with decreasing HVL. The EPOM shift did not change significantly with field size. Once the EPOM shift is applied, residual differences between the dose-to-water depth curves and dose-to-air cavity depth curves was found to be less than 0.5% in most cases. Significance: Monte Carlo calculations confirm the no EPOM shift for cylindrical ionization chambers in x-ray beams with HVLs greater than 0.5 mm Cu. The calculated EPOM shifts begin to deviate significantly from zero for HVLs less than 0.5 mm Cu. Current recommendations of a uniform zero EPOM shift for cylindrical ionization chambers in kilovoltage x-ray beams may be inadequate for accurate relative dosimetry in the entire medium-energy x-ray range.