Optically stimulated luminescence dosimeters versus thermoluminescence dosimeters accuracy in a PET facility

Objectives: Dosimetry is the measurement of radiation doses and is important to monitor personnel exposure levels to maintain the as low as reasonably achievable (ALARA) philosophy. Radiation dosimeters used to measure absorbed radiation are thermoluminescence dosimeters (TLD’s) or optically stimulated luminescence dosimeters (OSLD’s). Accuracy refers to how close data comes to the accepted value. Dosimeter readings must be accurate so radiation workers have a definitive assessment of exposure. The aim of this study is to compare the accuracy of TLD’s and OSLD’s under identical conditions.

Methods: One OSLD and one TLD were attached to the top of the Discovery VCT PET/CT scanner at UPMC Hillman Cancer Center and Washington Hospital (dosimeter set 1: four dosimeters comprising two of each type at each facility). After thirty patients, the dosimeters were removed and another pair (set 2) was placed for an additional thirty patients at each facility. Both facilities use [Fluorine-18]fludeoxyglucose FDG, and Hillman also uses [Gallium-68] dotatate. Key information from each scan included radiopharmaceutical and dose, bed number, and whether the CT was for attenuation or diagnostic purposes. The eight exposed dosimeters and control dosimeters were sent to Laundauer to be read.

Results: Although eight dosimeters were analyzed, only a report for set 2 was received. For comparison to the reported results, predicted baseline exposure values were calculated for each patient using the PET dosage, bed number, and exposure rate formula, and the results were summed for each set of thirty patients to produce total baseline values. The predicted baseline and reported values are detailed in Table 1 showing lower predicted values and higher OSLD vs. TLD results.

Table 1. Conclusions: The difference between predicted and actual values was unexpected. It was realized that the predicted values based only on PET dosages did not include the CT contribution or decay effects from injection time to scan time, both of which would have been difficult to calculate. Therefore, it is not possible from this study to make a definitive assessment of relative dosimeter accuracy. The higher OSLD values, however, are consistent with literature that OSLD’s are more sensitive than TLD’s. These results show that great care must be taken in the design of experiments to understand as much as possible the effect of input variables on predicted results, in this case the combined contribution CT dosages and radioisotope decay on the baseline PET predictions. Such understanding is important to ensure all critical experimental factors are accounted for.