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 an d the accuracy of this method, Rodshaped Teflon-imbedded CaSO4:Dy or L iF thermoluminescent dosimeters (TLDs) with dimensions 0.2 x 0.4 x 5 m m(3) were prepared from TLD-discs, To remove paraffin from the mini-TL Ds after cutting in a microtome the TLDs were Xylene-treated, which do es not affect the sensitivity, Irradiated mini-TLDs are sensitive to i llumination, Fading effects in darkness were examined after Co-60-irra diation at temperatures 4, 22 and 37 degrees C, For CaSO4:Dy mini-TLDs fading in air is small, The observed signal loss after implanting CaS O4:Dy mini-TLDs in gel and muscle tissue is the same at constant tempe rature and is increasing with the temperature, For LiF mini-TLDs the e ffect of signal loss in gel was smaller than for CaSO4:Dy dosimeters, For Co-60 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 CaSO,:Dy dosimeters the loss of sensitivity in gel is smaller at higher pHs. For LiF dosi meters 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 easi ly occur.