Fluence non-uniformity effects in air kerma determination around brachytherapy sources

Abstract

In ionization chamber dosimetry close to brachytherapy sources, the non-uniform photon fluence in the vicinity of the air cavity of the ionization chamber must be corrected for. In this paper, experimentally determined relative non-uniformity correction factors are determined for a PTW23331 chamber and a number of Farmer-type chambers. It is concluded that the 'anisotropic' theory of Bielajew agrees better with the experimental results than the 'isotropic' theory of Kondo and Randolph. The experiments show that neither the material choice nor the radius of the central electrode have any significant effect on the non-uniformity correction factors. Experiments on the wall-material dependence in the non-uniformity correction factor, which is predicted by the anisotropic theory, were inconclusive. Parameters for the Farmer-type chambers for calculation of theoretical non-uniformity correction factors are given. Data to derive anisotropic fluence non-uniformity correction factors are given in tabular form. An error in the Kondo and Randolph equations is reported and the corrected equations presented.

The non-uniformity correction factor for the cylindrical ionization chambers in dosimetry of an HDR192Ir brachytherapy source

“…performed an experimental study on the non-uniformity correction factor using various chambers of different radii and materials of the central electrode. [ 8 ] Their study supported the anisotropic behavior proposed by Bielajew at short distances. They also concluded that the choice of material and diameter of central electrode do not contribute significantly to the chamber's non-uniformity correction factors.…”

“…However, the high dose rate 192 Ir source used in this experimental study was line source of length of 0.35 cm. Tölli et al [ 8 ] derived a formula to average the theoretical values of point source formalism ( A pn ( r )) over the length of the source. They found that the averaging increased the theoretical values by approximately 0.5 and 0.3% relative to point-source formalism at a distance of 0.8 and 1.0 cm respectively.…”

“…The parameters (r) and A′ pn (r) for the calculation of the non-uniformity correction factor for cylindrical chambers are given in literature. [ 8 ] These parameters are given in table as a function of the cylindrical chamber's shape factor, σ = R c / L c and the distance factor, α = R c / r , where R c is the chamber's internal radius and L c is the internal half-length of the chamber and r is the measurement distance.…”

“…The air kerma readings taken at different measurement distances (more than five) for more than 10 cm were corrected for the non-uniformity correction factor. [ 8 10 ] The measured air kerma M d has contribution from the scattered radiation M S . The correction for the room scatter[ 12 ] was made in order to know the measured air kerma reading from the primary radiation, i.e., M u = M d - M S .…”

Response Characteristics of a Cylindrical Ionization Chamber to the Low- and Medium-Energy X-ray Beams

“…Many authors calculated the ionization chamber response using Monte Carlo codes [1][2][3][4][5][6][7] . Most of the authors focused on the evaluation of the ionization chamber response to relatively high-energy photons such as 60 Co gamma-rays.…”