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 ov erall correction factor k(ch) for in-phantom dose determinations in or thovoltage 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 ener gies combined with Monte Carlo calculations; (2) a displacement effect and (3) a stem effect, both calculated using the Monte Carlo method f or 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 e nergy) when going from 2 cm to 5 cm for a small circular 20-cm(2) fiel d. In the medium-energy range (greater than or equal to 100 kV), varia tions are limited to, at most, 1.5% for 120 kV-150 kV when comparing t he most extreme variations in field size and depth (i.e., 2-cm depth; 20-cm(2) area compared to 5 cm depth; 200-cm(2) area). Depth variation s most importantly affect the overall correction factor by hardening o f the photon fluence spectrum, whereas field diameter variations affec t the factor by increase or decrease of contributions of photon scatte ring. The work shows that taking into account the uncertainties adopte d in the recent review of data and methods recommended in the IAEA cod e of practice, the dependence of the overall correction factor on dept h 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-m m Cu, average energy: 144 keV). (C) 1996 American Association of Physi cists in Medicine.