Radial dose distribution of 192Ir and 137Cs seed sources.

The radial dose distributions in water around 192Ir seed sources with both platinum and stainless steel encapsulation have been measured using LiF thermoluminescent dosimeters (TLD) for distances of 1 to 12 cm along the perpendicular bisector of the source to determine the effect of source encapsulation. Similar measurements also have been made around a 137Cs seed source of comparable dimensions. The data were fit to a third order polynomial to obtain an empirical equation for the radial dose factor which then can be used in dosimetry. The coefficients of this equation for each of the three sources are given. The radial dose factor of the stainless steel encapsulated 192Ir and that of the platinum encapsulated 192Ir agree to within 2%. The radial dose distributions measured here for 192Ir with either type of encapsulation and for 137Cs are indistinguishable from those of other authors when considering uncertainties involved. For clinical dosimetry based on isotropic point or line source models, any of these equations may be used without significantly affecting accuracy.