In vivo absorbed dose measurements with mini-TLDs--parameters affecting the reliability.

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.