Anisotropy of the Radiation Field Following Canine Sn-117m Treatment

Tin-117m (117mSn) is used to treated dogs with osteoarthritic joints by radiosynoviorthesis. The internal conversion and Auger electrons emitted by the 117mSn provide the therapeutic effect. Sn-117m also emits gamma rays, of which the most significant is 158.6 keV. The external radiation field around a treated dog is of interest to limit the dose to the owners/caretakers of the dog. The dog's torso attenuates the radiation being emitted toward the opposite side of the dog's body. This leads to a radiation field that is significantly non-isotropic. This study characterizes the anisotropy of this field to permit maximum dose rate measurements to be used to calculate the dose to individuals in the vicinity of the dog. Measurements were made in nine directions and at two distances, 0.3 and 1.0 m, to characterize common distances and spatial orientations for human-dog interactions. From these measurements, the percent reduction in the average dose rate compared to the maximum dose rate was determined. From a radiation safety perspective, the important factor is the minimum amount of shielding effectiveness or percent reduction that can be expected. A reasonable measure for this value is the fifth percentile of the shielding effectiveness distribution. The fifth percentile shielding effectiveness measures are 27% and 21% at 0.3 and 1.0 m, respectively.