PARAMETERS TO CONSIDER FOR MEASUREMENTS OF ABSORBED DOSES IN-VIVO WITH MINI-THERMOLUMINESCENT DOSIMETERS

Background. For systemic radiation therapy, i.e., radioimmunotherapy, there is a demand for direct methods of measuring the absorbed dose in vivo. One such method is the use of mini-thermoluminescent dosimeters (TLDs). This paper reports an investigation of the sensitivity of tis sue implanted mini-TLDs (calcium sulfate:Dy, 0.2 X 0.4 X 5.0 mm). Meth ods. After being irradiated with cobalt-60, the mini-TLDs were left fo r as long as 9 days in air, gel, and muscle tissue. Results. There was an extensive signal loss, which increased with time, except in air. A fter 9 days in gel or muscle tissue at room temperature, the signal wa s decreased to one third of its original value. The dosimeters needed to be kept in constant darkness. There was a strong pH dependence, wit h a loss of sensitivity of 63% at a pH below 5, which got smaller at h igher pH values and reached 10% at pH = 10. Conclusions. When using mi ni-TLDs in vivo, one must calibrate the dosimeters in similar milieus, unless the position of the dosimeters in tissue after implantation ca n be monitored for temperature, pH, and liquid flow.