The effect of air cavity size in cylindrical ionization chambers on the measurements in high-energy radiotherapy photon beams--an experimental study

The present investigation is a continuation of a previous study on the effect of the diameter of the air cavity in cylindrical ionization chambers on perturbation correction factors. Measurements were made using high-energy radiotherapy photon beams (4, 6 and 15 MV) in a water phantom. Two different pairs of cylindrical ionization chambers were used. The chambers in each pair had identical materials and construction but different air cavity diameters. The same methods were employed as in our previous investigation. The diameter of the air cavity in cylindrical ionization chambers influences the mass ionization (the?measured ionization expressed per unit mass of air in the chamber air cavity) at the depth where the maximum ionization is observed and a normalization at this depth is therefore not correct. The corrections obtained at depths of 50 and 100?mm in the phantom showed that the air cavity diameter in cylindrical ionization chambers has a greater effect on the perturbation effects than the photon beam quality. The corrections found at depths of 50 and 100?mm are smaller than those currently used in dosimetry protocols.