An investigation has been carried out on the factors which affect the absolute calibration of thermoluminescent dosimeters (TLDs) used in beta particle absorbed dose evaluations. Four effects on light output (LO) were considered: decay of detector sensitivity with time, finite TLD volume, dose linearity, and energy dependence. Most important of these was the decay of LO with time in culture medium, muscle tissue, and gels. This permanent loss of sensitivity was as large as an order of magnitude over a 21-day interval for the nominally 20-microns-thick disc-shaped CaSO4(Dy) TLDs in gel. Associated leaching of the dosimeter crystals out of the Teflon matrix was observed using scanning electron microscopy. Large channels leading from the outside environment into the TLDs were identified using SEM images. A possibility of batch dependence of fading was indicated. The second most important effect was the apparent reduction of light output due to finite size and increased specific gravity of the dosimeter (volume effect). We estimated this term by calculations as 10% in standard "mini" rods for beta particles from 90Y, but nearly a factor of 3 for 131I beta particles in the same geometry. No significant nonlinearity of the log (light output) with log (absorbed dose) over the range 0.05-20.00 Gy was discovered. Energy dependence of the LO was found to be not detectable, within measurement errors, over the range of 0.60-6.0 MeV mean energy electrons. With careful understanding of these effects, calibration via gel phantom would appear to be an acceptable strategy for mini TLDs used in beta absorbed dose evaluations in media.(ABSTRACT TRUNCATED AT 250 WORDS)
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 tissue implanted mini‐TLDs (calcium sulfate:Dy, 0.2 × 0.4 × 5.0 mm).
Methods. After being irradiated with cobalt‐60, the mini‐TLDs were left for 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. 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.
Conclusions. 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. Cancer 1994; 73:985–8.
Mini-TLDs have been proposed and widely used for in vivo measurements of absorbed doses in radionuclide therapies. The present investigation reports in detail on the signal dependence on different parameters and the accuracy of this method. Rodshaped Teflon-imbedded CaSO4:Dy or LiF thermoluminescent dosimeters (TLDs) with dimensions 0.2 x 0.4 x 5 mm3 were prepared from TLD-discs. To remove paraffin from the mini-TLDs after cutting in a microtome the TLDs were Xylene-treated, which does not affect the sensitivity. Irradiated mini-TLDs are sensitive to illumination. Fading effects in darkness were examined after 60Co-irradiation at temperatures 4, 22 and 37 degrees C. For CaSO4:Dy mini-TLDs fading in air is small. The observed signal loss after implanting CaSO4:Dy mini-TLDs in gel and muscle tissue is the same at constant temperature and is increasing with the temperature. For LiF mini-TLDs the effect of signal loss in gel was smaller than for CaSO4:Dy dosimeters. For 60Co external irradiation supralinearity already starts between 0.5 and 1 Gy for both kinds of dosimeter material. There is a strong pH dependence of the signals from the mini-TLDs. For CaSO4:Dy dosimeters the loss of sensitivity in gel is smaller at higher pHs. For LiF dosimeters the loss of sensitivity is smallest for neutral pH. We conclude that using mini-TLDs for in vivo dosimetry requires careful handling and proper calibration for accuracy in the measurements. Without such calibration errors exceeding 65% for CaSO4:Dy and 40% for LiF may easily occur.
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