[Dosimetry of a blood irradiator]

Blood and blood products are irradiated to avoid the graft-versus-host disease (GVHD) in immunosuppressed patients and to destroy tumor cells during the intra-operative autotransfusion in tumor surgery. For that purpose more and more dedicated gamma irradiators are used. In most cases the equipment is supplied with a dose calibration factor for a totally filled irradiation canister. As users handle different blood product volumes, it is necessary to investigate the influence of the irradiated blood volume on the absolute dose in a reference point and the dose distribution in the irradiation volume.

The dose rate in the center of an empty irradiation canister of an IBL 437C blood irradiator (CIS Diagnostic) was investigated by means of Fricke solution dosimeters from the Physikalisch-Technische Bundesanstalt (PTB). Using thermoluminescence dosimetry (TLD) this value could be transferred to a situation with an empty or completely filled respectively with 2 blood samples (270 ml each) filled canister. Also essential for the irradiation of blood is the knowledge of the dose distribution in the irradiated volume. The distributions in the empty and the realistic filled canister were measured by positioning the TLD on the plexiglas holder in a regular pattern. The case of a completely filled container was investigated by means of the MR Fricke gel dosimetry. All distributions are presented as dose-volume-histograms (DVH).

The TLD-measurement in the center of the completely filled canister yielded a 4.8% higher dose rate value as compared to the suppliers certificate. From the investigations using the Fricke solution dosimeters in air combined with TLD-measurements values for the complete bandwidth of different container fillings could be derived. So the dose rate in the centre of the canister in the boundary conditions empty and full canister as compared to the values for the realistic filling condition (2 bags) are 117.5% and 94% respectively. Axial dose distributions and DVH have been determined for the 3 filling conditions.

We recommend a dose calibration measurement of a blood irradiator to determine the irradiation times for the chosen filling condition, which is typical for the hospital. The DVH presented in this work can be used to derive a value for the dose variance within the irradiated blood.