Radiol Dose Distribution of ¹⁹²Ir and ¹³⁷Cs Seed source

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.