Dependence of overall correction factor of a cylindrical ionization chamber on field size and depth in medium-energy x-ray beams.

In this paper we examine the depth and field size dependence of the overall correction factor kch for in-phantom dose determinations in orthovoltage x-ray beams. The overall correction factor is considered to be composed of three contributions, i.e., (1) a contribution from the angular dependence of the chamber response free-in-air, derived based on the measured directional response of the NE2571 for different energies combined with Monte Carlo calculations; (2) a displacement effect and (3) a stem effect, both calculated using the Monte Carlo method for different field sizes and depths. The results show a variation of, at most, 2.2% at the lowest photon energies (29.8-keV average photon energy) when going from 2 cm to 5 cm for a small circular 20-cm2 field. In the medium-energy range (> or = 100 kV), variations are limited to, at most, 1.5% for 120 kV-150 kV when comparing the most extreme variations in field size and depth (i.e., 2-cm depth; 20-cm2 area compared to 5 cm depth; 200-cm2 area). Depth variations most importantly affect the overall correction factor by hardening of the photon fluence spectrum, whereas field diameter variations affect the factor by increase or decrease of contributions of photon scattering. The work shows that taking into account the uncertainties adopted in the recent review of data and methods recommended in the IAEA code of practice, the dependence of the overall correction factor on depth and field size is insignificant for the radiation qualities between 100 kV (HVL 0.17-mm Cu, average energy: 52 keV) and 280 kV (HVL 3.41-mm Cu, average energy: 144 keV).