Parameters to consider for measurements of absorbed doses in vivo with mini-thermoluminescent dosimeters.

Abstract

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 tissue implanted mini-TLDs (calcium sulfate:Dy, 0.2 x 0.4 x 5.0 mm). After being irradiated with cobalt-60, the mini-TLDs were left for as long as 9 days in air, gel, and muscle tissue. There was an extensive signal loss, which increased with time, except in air. After 9 days in gel or muscle tissue at room temperature, the signal was decreased to one third of its original value. The dosimeters needed to be kept in constant darkness. There was a strong pH dependence, with a loss of sensitivity of 63% at a pH below 5, which got smaller at higher pH values and reached 10% at pH = 10. When using mini-TLDs in vivo, one must calibrate the dosimeters in similar milieus, unless the position of the dosimeters in tissue after implantation can be monitored for temperature, pH, and liquid flow.