Validation of radiation dose received by frozen unprocessed and processed bone during terminal sterilisation

Fresh frozen femoral heads (FH) and frozen processed bone (FP) are widely used as a source of allograft bone. The FP bone and some of the FH are terminally sterilised by the National Blood Service Tissue Services (NSBTS), via application of a minimum 25 kGy gamma radiation dose. To comply with the Guidelines for the Blood Transfusion Services in the United Kingdom (2002), frozen musculoskeletal tissue must be maintained below -40 degrees C during storage and transit. In practice, NBSTS stores bone long-term in -80 degrees C freezers. During transport for irradiation, a temperature of circa -79 degrees C is maintained by packing the bone in dry ice. An evaluation of the radiation dose received by bone has previously been made via dosimeters located within the tissue and dry ice, however, some evidence suggests that low temperature can influence the accuracy of the dosimeter readings. The aim of this study was to determine the actual radiation dose received by FH and FP bone during the irradiation process. This was accomplished by comparing radiation dose readings from dosimeters placed in dry ice with dosimeters placed in a dry ice substitute of similar dimensions and density i.e., polytetrafluoroethylene (PTFE) at ambient temperature. New packing formats were developed for both FH and FP bone such that 15 FH or 3 kg of FP bone could be irradiated in one transport box at any given time in a standardised fashion. The data show that low temperature consistently increased dosimeter readings 10--27%, and that radiation dose always fell within the range of 25--40 kGy (FH=25.1--35.7 kGy; FP bone=25.2--32.4 kGy).