Answer to Question #13286 Submitted to "Ask the Experts"

What is the use of a TLD reader in a radiotherapy department in a hospital? Also, what is the difference between manual and automatic TLD readers?

Thermoluminescent dosimeters (TLDs) have been used for many years in many areas of radiation dosimetry, perhaps most notably in personnel dosimetry in which occupational workers are badged with a TL device to monitor external radiation doses. The use in radiotherapy has also been common. The TLDs have been and are often used in quality assurance/quality control programs in which the TLDs are mounted in a particular phantom, often fabricated of plastic polymethylmethacrylate, to establish or verify delivered doses. When photon beams are used, the amount of phantom material covering the TLDs is generally selected to ensure secondary charged particle equilibrium for the photon energies being used, although in some instances the bare dosimeter may be used to estimate the actual entrance surface dose under conditions of non-equilibrium. For therapeutic electron beams, sufficient material extends in all directions to ensure an equilibrium with respect to scattered electrons at the dosimeter location. TLDs have also been used in studies with phantoms to evaluate doses at various depths to simulate doses to particular tissues in the body. There have also been studies done in which TLDs, typically covered with an equilibrium thickness of appropriate tissue-equivalent material, mounted on the surfaces of patients being treated with therapeutic beams have been used to establish entrance doses for specific locations. The most commonly used TL material has been TLD-100, LiF(Mg,Ti).

In recent years an alternative external dosimetry technique, using optically stimulated luminescence dosimeters (OSLDs), has gained considerable favor. Optically stimulated luminescence is similar to thermoluminescence except that light is used instead of heat to release trapped electrons from their excited level trapping sites in the crystalline active matrix. The most commonly used material for OSL has been Al₂O₃(C). At least one vendor provides OSL devices in small volumes that are embedded in thin adhesive films that may be attached to most any part of the body such that monitoring of surface doses or doses at selected depths, if added tissue-equivalent material is used, may be readily accomplished. Readout is designed to be very fast and convenient, an added fact that has greatly enhanced the popularity of this technique in monitoring both therapeutic and some diagnostic procedures, most especially some fluoroscopic procedures.

The major difference between a manual reader and an automatic one is that the manual unit may accept only a single dosimeter at a time, although some may read out more than one active element contained within a single dosimeter. Each dosimeter to be read out must be individually inserted and read. The automatic units allow loading of multiple dosimeters into the unit. Some units will accept hundreds of dosimeters, which are automatically sequentially processed with data readout information being collected and correlated with the respective dosimeters.

I hope this is helpful.

George Chabot, PhD, CHP