Finger doses received during 153Sm injections

This study was undertaken to determine the dose received by the skin of the fingers of clinical and laboratory staff during injections of 153Sm. The use of 153Sm, chelated with ethylenediaminetetramethylene phosphonic acid (153Sm-EDTMP), is coming into more frequent use in radionuclide therapy since its approval by the U.S. Food and Drug Administration in March 1997. 153Sm emits a range of medium-energy therapeutically useful beta particles that have been found beneficial in the palliation of metastatic bone cancer pain. It also emits a range of gamma rays. Calculations have been undertaken to provide the beta-particle and gamma-ray dose rates, at a depth within the skin corresponding roughly to the basal cell layer, when the finger is placed in direct contact with the external surface of a syringe containing 153Sm. The beta-particle dose rates were modeled using an empirically based Monte Carlo approach previously described by Beddoe and Kelly. The gamma-ray dose rates were modeled using a distributed point source approach previously reported by Pattison et al. In the calculations it is assumed that a typical administered activity is 2.6 GBq, with a finger-syringe contact time of 30 s. The skin dose, due to both beta particles and gamma rays when the finger is centrally placed over an active volume of 0.3 mL in a 1-mL syringe, is calculated to be 77 mGy per injection. Similarly, for an active volume of 1.0 mL in a 2.5-mL syringe, the dose is calculated to be 10 mGy per injection. In view of the ICRP recommended weekly skin dose limit of 10 mGy, both of the above two doses are excessive. If, however, the fingers are placed at the rear end of the syringe barrel, where they are only exposed to the gamma rays, the above two doses are reduced to 0.069 and 0.139 mGy per injection, respectively. Both of these two doses are well within the recommended weekly dose limits for the skin. It is found that the weekly dose limit for the skin is readily exceeded if the fingers are in direct contact with the external surface of the syringe and located over the active volume. However, if handled at the rear end of the syringe barrel, a typical weekly workload can be managed without exceeding the recommended dose limits.