Comparison of two independent exposure measurement techniques for clinical I-125 seeds

This investigation contains the results of exposure measurements on two types of clinical I-125 seeds: a model 6702 no-marker seed, and a model 6711 silver wire seed. Measurements were made locally with a parallel-plate mammography chamber, traceable to the National Bureau of Standards (NBS) calibration. The same seeds were also measured at NBS using a reentrant chamber that was developed and calibrated there. The mammography chamber measurements showed an approximately 5% higher exposure rate for the no-marker seed than the reentrant chamber measurements. On the silver wire seed, good agreement was obtained between the two techniques. It was found that exposure rate variations in the plane perpendicular to the long axis of the seed (or seed axis) were substantial, due in part to the nonsymmetric distribution of radioactive material in the seed.