In Vivo Absorbed Dose Measurements with Mini-TLDs Parameters affecting the reliability

Abstract

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 × 0.4 × 5 mm³ were prepared from TLD-discs. To remove paraffin 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×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 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.