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Literature Review on Thermoluminescent Dosimetry

Introduction

Thermoluminiscent dosimetry is a method used for a number of applications in radiotherapy. Some of these applications are mailed dosimetry and measurements in phantoms. Thermoluminiscent dosimeters are also important in applications that require small dosimeters. One disadvantage of these dosimeters is the absence of on-line readout during treatment.

Theory of Operation

The thermoluminescent dosimetry operation is based on the fact that when electrons in some materials are provided with enough energy via heating the material, convert from unstable to metastable states. Through this procedure, the electrons release an optical photon that can usually be detected and its light output measured by a photomultiplier. The readout procedure partially returns the dosimeter to its original state. However, any residual metastable electrons may possibly be eliminated by additional heating of the material at a higher temperature. This heating process is called annealing and it enables the dosimeters to be indefinitely reused (International Atomic Energy Agency, 2013).

Methods of Storing and Readout of Dose Information

Thermoluminescent dosimetry materials can take several chemical and physical states. In radiotherapy, the most commonly used material is lithium fluoride smeared with traces of magnesium and titanium ( LiF: Mg,Ti). The materials were manufactured as emitted ribbons cut into pieces of approximately 3mm square and 0.9mm width. This material is stored as TLD 100 and it contains naturally abundant lithium. There is also an almost similar material that comes in different sizes and shapes. This material is made of lithium fluoride smeared with phosphorus and small amounts of magnesium and copper (Lif: Mg,Cu,P) and is more sensitive than the former. It also less sensitively depends on radiation energy.

It is greatly sensitive to inaccurate thermal treatment thus it is more advantageous to use the former for routine radiotherapy. Lithium fluoride was once utilized in powder form. However, other forms have been introduced such as a mixture of lithium fluoride discs and Teflon. The mostly used forms are chips, which is any compressed form of dosimeter, these chips could take form of discs and micro rods. It is highly important for TLD materials not to be contaminated and thus only vacuum tweezers are used when handling them. If contamination occurs, chips should be cleansed using pure alcohol and rinsed in deionized water. Trays or planchettess should be regularly cleaned since they may alter the measured light output if contaminated. Alteration in reflectivity between trays pose a threat especially in multiple planchette readers (International Atomic Energy Agency, 2013).

There are two requirements needed for readout of dose information of TLD sample. These include mode of measuring light output and an efficient form of heating. In one method, the dosimeter is placed on a tray, pushed into the reader and then heat is brought into close contact under the planchette. Another way is heating dosimeter using hot gas. In the former method, any form of dosimeter can be read and can more precisely control the heating cycle. The latter mode is faster and is independent on the thermal contact degree with the dosimeter. A photomultiplier is used to measure the lighting output of dosimeter. The light output varies with increase in temperature of dosimeter and the voltage applied to the photomultiplier will affect the reading (International Atomic Energy Agency, 2013).

Dosimetry Properties

The linearity of light output: light output of lithium fluoride smeared with magnesium and titanium (Lif: Mg,Ti) has a linear relation to dose up to approximately 1 Gy after which it becomes supralinear. For LiF: Mg,Cu,P its response is linearly related up to 10Gy after which it also becomes supralinear. For a certain dosimeter whose annealing cycle is consistent, the linearity curve remains stable over its lifetime. However, it is advised to confirm this characteristic from time to time (International Atomic Energy Agency, 2013).

Energy beam and quality dependence of dosimeters: Lithium fluoride measurements seem to more easily convert to dose that ion chamber measurements. The use of beam quality is the simplest procedure to follow when calibrating chips against ionization chamber. However, correction factors will increase if thicker dosimeters are utilized or if stacked up. Additionally, it is considered highly important to dosimeters in the relevant beam especially when X-rays energies of less than 300kV are used. The coefficient of calibration is expected to follow the changes in coefficient ratio of mass energy. At such energies, LiF: Mg,Cu,P is a better dosimeter to apply (International Atomic Energy Agency, 2013).

Angular dependence of dosimeter: This kind of dependence can be disadvantageous especially because the chips orientation can greatly affect the cavity size. This scenario is more relevant for electrons and brachytherapy where there may be a higher doe gradient. However, it makes more sense to irradiate normal to the flat face of chips or to the long axis of micro rods. Angular dependence of TLD result from attenuation and scatter in the buildup cap (International Atomic Energy Agency, 2013).

Fading: It is where thermoluminescent signal between irradiation and readout decreases and is usually caused by electrons in the lower energy traps changing to stable state; it affects lower temperature traps. Therefore, it is highly relevant for the readout process to exclude lower thermoluminescnet signal. Unless an appropriate readout cycle is introduced, fading can be minimized by waiting for at least an hour before reading dosimeters

Dosimeters background signals: Normally, background signals can never be greater than 1mGy. A background signal arises from dark current of photomultiplier and the residual signal from previous irradiation. When exposed to light, the dark current increases but may take time to stabilize after switching on amplifier. Therefore, reader should be left open. Residual signals arising from previous irradiations may not pose a threat if sufficient annealing cycle is used. Under natural circumstances, in absence of radiation, TLD materials tend to produce a small luminescence signal. A chemical reaction on dosimeters surface may be associated with this production. This can be reduced by doing readout at an atmosphere with high purity nitrogen. When running read cycle with no dosimeter, it is possible to differentiate reader associated background from inherent TLD background (International Atomic Energy Agency, 2013).

Annealing: It is a process where dosimeter is maintained at a higher temperature and giving them time to cool in a controlled manner. The time taken to cool determines the sensitivity of the dosimeter, with rapid cooling resulting to higher sensitivity. Thus, the annealing cycle should be reproducible. It is important to grant one batch of chips the same heat treatment. It can be done by placing all the dosimeters on a tray made of stainless steel, or anodized aluminium. The metal tray will equally conduct heat thus leading to the uniformity of the heat treatment (International Atomic Energy Agency, 2013).

Glow curves: They are representation of photomultiplier output against the temperature of dosimeter. Many modern readers possess a built in computer interface that allows storing and display of glow curves. This equipment is relevant as it permits retrospective analysis of the glow curve in case of readings that are not expected. There are three phases typically associated with the heating cycle. These are the low temperature pre-readout cycle which eliminates low temperature peaks; the measurement phase which integrates light output from main peaks; and high temperature post readout phase. The glow curve determines whether readout cycle is appropriate or not, hence its relevance. However, low temperature peaks should not be included in the integration while high temperature peaks should be totally be included.

Clinical Application in Radiotherapy, Imaging or Radiation.

Radiotherapy often depends on dosimetry for application of cancer treatment and reduction or total avoidance of severe cases of toxicity in patients. Dosimetry has been hit with new challenges with small and changing radiation fields being at the center of most of these applications, for instance stereotactic ablative body radiotherapy. A safe radiotherapy procedure will require dosimetry for instance detection of dose in a phantom. The accelerator beam characteristics will have to be considered, both as input information for planning treatment and as determine on how great the calculation algorithm works. Dosimetry is carried out in the end to end testing. A single point measurement is mostly used to carry out absolute dosimetry, on the other hand, ionization chamber is always an optional method (Kron, Lehmann, & Greer, 2016).

Film dosimetry, was established as a vital tool for verification of radiotherapy treatment and quality assurance. Film dosimetry stands out because it is able to integrate a dose in any plane and utilize it in any desirable phantom. Detection of dose relies on a light attenuation that is measured per film pixel. The amount of radiation received at that particular pixel determines the dose magnitude. However, film dosimetry has issues some of them being its sensitivity and accuracy which are relative to an ideal water detector. Film dosimetry needs steps of film processing and film readout systems which can impact accuracy and sensitivity. Once upon a time, radiographic film was applied, but nowadays this has been almost totally replaced by radio chromic film types (Devic, 2011)

Brachytherapy is mostly used to treat skin and breast cancer. This is where radioactive sources are placed in close contact with the cancerous tumor. The small size of these sources allow them to be easily inserted into or onto the tumors. It is at this point when radiation is emanated from these sources interact and kill neighboring tumor cells. The radioactivity in the sources decays, releasing poisonous radiation to cancer cells until not radiation can be detected in the sources. The radioactivity sources are never removed although they remain harmless to the patient’s life. In other cases, radioactive sources of longer half-life are removed from the tumors after successful administration of prescribed dose of radiation to cancer cells (Hill, Healy, Holloway , Kuncic, Thwaites, & Baldock, 2014).

Kilo voltage X-ray beams are also used in treatment of skin cancer since one of their property is that maximum dose happens very close to the surface. Mostly, radiotherapy is done after the tumor has been surgically removed and radiation is applied to remove and cancer cells remaining within the tumor. During the treatment of both non AIDS and AIDS related Kaposi-sarcoma, therapeutic kilo voltage X-rays beams may be applied. Additionally, kilo voltage X-ray beams have been increasingly used in intra operative units, inside small animal irradiators as well as linear accelerators. However, Kilo voltage X-ray have a number of challenges. Firstly, the presence of relatively big dimension in the depth direction of many dosimeter.

This means that there can be a significantly great dose gradient over the dosimeter measurement due to the deep rapid fall off of dose. Another problem is that the dosimeter response is sensitive to the materials utilized during its construction. Lastly, cavity theory can never be used for reference since ionization chambers do not pose as Brag-ray cavities in kilo voltage X-rays. It is vivid that dosimetry of imaging kilo voltage beams have become more relevant since the great planar and cone beam CT imaging systems have become widely available (Hill, Healy , Holloway , Kuncic, Thwaites, & Baldock, 2014).

References

International Atomic Energy Agency. (2013). Development of procedures for in vivo dosimetry in radiotherapy. IAEA Human Health Report No. 8.

Hill, R., Healy, B., Holloway, L., Kuncic, Z., Thwaites, D., & Baldock, C. (2014). Advances in kilovoltage x-ray beam dosimetry. Physics in Medicine & Biology, 59(6), R183.

Kron, T., Lehmann, J., & Greer, P. B. (2016). Dosimetry of ionising radiation in modern radiation oncology. Physics in Medicine & Biology, 61(14), R167.

Devic, S. (2011). Radiochromic film dosimetry: past, present, and future. Physica medica, 27(3), 122-134.

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