Dosimetric measurements were performed to characterize a new I-125 source t
hat is a variant design of an existing source, designated as MED3631-A/S, a
nd that has application in interstitial brachytherapy. The new source, desi
gnated as MED3631-A/M, has centralized radio-opaque markers. In the origina
l MED3631-A/S source, the radio-opaque markers are separated. Thermolumines
cent dosimeters were placed in phantom to measure transverse axis and angul
ar dose profiles over a range of distances from 0.5 to 7 cm. The data were
analyzed in terms of parameters recommended by AAPM Task Group No. 43. Tabu
lar data evaluated in liquid water are provided for the dose-rate constant,
Lambda, radial dose function, g(r), the anisotropy function, F(r, theta),
the anisotropy factor, phi(an)(r), the point-source approximation anisotrop
y constant, <(phi)over bar>(an). The dose-rate constant was determined by a
n absolute method using a Cobalt-60 reference and by relative measurements
using calibrated I-125 source(s). Values of the dose-rate constant are prov
ided for both the 1985 and 1999 NIST air-kerma strength standards. The new
source is comparable to both the MED3631-A/S and the model 6702 I-125 sourc
e designs, demonstrating equivalent radial dose function, g(r). Differences
in the value of the dose-rate constant, Lambda, and the anisotropy of the
dose distributions in phantom are discussed in light of the improved isotro
py of the new design, the MED3631-A/M source, and the uncertainty involved
in the dose measurement using a Cobaltd-60 reference. (C) 1999 American Ass
ociation of Physicists in Medicine. [S0094-2405(99)02009-X].