Mini-TL-Dosimeters for in vivo measurements

Background: Mini-TLDs are small rods of Teflon with CaSO4;Dy or LiF as dosimeter material. Their dimensions are 0.2x0.4x5 mm. They were produced from ordinary TLD-discs. Professor Barry Wessels, Washington, invented the mini-TLD-method and used the dosimeters for absorbed dose measurements in different tumor models and animal experiments. However, he did not examine the different properties of these mini-TLDs. Aim: We want to investigate different parameters of mini-TLDs, such as fading in daylight and darkness, signal loss in tissue, pH-dependence and supralinearity. Results: For CaSO4:Dy mini-TLDs we found that fading in air in darkness was small. For LiF mini-TLDs the signal increased up to at most 135%. The observed signal loss after implanting mini-TLDs in gel and muscle tissue is considerable. For CaSO4:Dy mini-TLDs the signal decreased to 32% of its original value after 9 days at 22 deg. C. The corresponding value for LiF is 65%. The signal losses in gel and muscle tissue are the same at one temperature and are increasing with the temperature. The results after pretreating mini-TLDs with liquid or gel still show signal losses, although to a lesser extent. The explanation to those signal losses must be that TLD-crystals are dissolved by the material. This has been visualized by SEM- and μCT-pictures of treated dosimeters. The pH-dependence of the TL-signal is high. For external irradiation, supralinearity already starts around 1 Gy for both kinds of dosimeter materials. An important factor affecting the TLD-signal is the energy distribution of photons in the media. Monte Carlo simulations show that a substantial part of the photon fluence from a photon emitting activity in vivo is scattered low energy photons affecting the signal of the CaSO4:Dy dosimeters. Consequence: To be able to use mini-TLDs for in vivo measurements, one has to make corrections for the above mentioned parameters or calibrate the dosimeters i a similar milieu. Mini-TLDs can be used to e.g. verify existing dose planning systems