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Dosimetric Control with Thermoluminescent Dosimetry (TLD)

Different type of the in-vivo dosimetry is the only tool for checking the dose actually delivered to the patient. The detection of various types of errors in the dose delivery process can improve the quality of patient care. Various techniques are used for in vivo dosimetry such as diodes, thermoluminescent dosimetry (TLD), films and electronic portal imaging devices. Types of in vivo dosimetry measurements, such as entrance, exit, intracavitary and surface dose measurements are defined and well described in several different protocols.

Dosimetric control with TLDs

TLDs are based on the principle that imperfect crystals can absorbed and store the energy of the ionising radiation. With heating, this energy is emitted in the form of electromagnetic radiation, in the visible wavelength region; this process is detected by photomultiplier and correlated with the absorbed dose by the material. Approximately 1% of the energy deposited as absorbed dose in the TL material is emitted as light when the material is heated, providing the dosimetric parameter to be measured. Most commonly used TL detectors are: LiF:Mg,Ti (lithium fluoride), Li2B4O7:Mn (lithium borate), CaSO4:Dy (calcium sulphate), etc. TLDs are available in different form of powder or small dimension of solid states dosimeters, such as rods, chips, etc.A typical cycle in the TLD reader consists of a preheat phase, without light emission, a read out period with glow curve for dose measurement, annealing period without light integration and the cooling-down period. During the measurement we receive the glow curve, which is the graphical representation of the emitted light intensity that increases with the increasing temperature. The peaks in the glow curves may be correlated with trap depths responsible for thermoluminescence emission. TLDs need to be calibrated before they are used. To derive the absorbed dose from the TL reading a few correction factors have to be applied, such as the energy and dose response non-linearity.

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