DOSIMETRIC CHARACTERISTICS OF A DOUBLE-WALL I-125 SOURCE FOR INTERSTITIAL BRACHYTHERAPY

Recently, a newly designed encapsulated source of I-125 has become com mercially available for use in permanent and temporary interstitial br achytherapy. The I-125 sources in current use come in two different co nfigurations: the Model 6711 source (Medi Physics/Amersham) for perman ent implants has radioactive iodine adsorbed on the surface of a silve r wire, and the Model 6702 (Medi Physics/Amersham) source for temporar y implants has radioactive iodine absorbed in three spherical resin ba lls. Both of these iodine sources are encapsulated in a thin-walled sh ell (0.05-mm thick) made of titanium. The newly designed I-125 source (Best Industries Model 2300 series) contains radioactive iodine adsorb ed on a tungsten wire that is encapsulated by two walls of titanium. T his double-walled I-125 source offers the following potential advantag es: (i) Because it contains radioactive iodine on the ends as well as the circular surface of the tungsten wire, it can produce a more isotr opic dose distribution than the sources in current use; (ii) because i t is available in a wider range of source strengths, it is suitable fo r both temporary and permanent implantation; (iii) because it has a tu ngsten radiographic marker, source localization is considerably easier than the I-125 Model 6702 source that has no radiographic marker; and (iv) because it uses a double-walled encapsulation the risk of radioa ctive contamination due to source rupture is considerably reduced. In this work, dose distributions produced by the new design I-125 source (Model 2300) for interstitial brachytherapy have been measured using L iF TLD's in a Solid Water phantom. Dosimetric characteristics of the n ew I-125 sources are compared with those of the currently available I- 125 sources. Radial dose function for the Model 2300 source is found t o be similar to that for the 6702 source, as expected by the lack of s ilver characteristic x rays in the photon spectrum from the 2300 sourc e. Using the calibration of source strength based upon the 6702 standa rd, the dose-rate constant for the 2300 source was determined to be 0. 86 cGy h-1 U-1 [equal to 1.10 cGy h-1 mCi-1 (app)]. From the measured two-dimensional dose distributions around the source, the anisotropy f unction for the new source was determined as a function of radial dist ance and angle. The dose distribution produced by the Model 2300 sourc e was considerably more isotropic than those produced by the 6711 and 6702 sources.