J. Seuntjens et F. Verhaegen, DEPENDENCE OF OVERALL CORRECTION FACTOR OF A CYLINDRICAL IONIZATION-CHAMBER ON FIELD SIZE AND DEPTH IN MEDIUM-ENERGY X-RAY-BEAMS, Medical physics, 23(10), 1996, pp. 1789-1796
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.