BEAM MODIFYING DEVICES and TLD badge.pptx

beam modifying devices and TLD batch

1. BEAM MODIFYING DEVICES &TLD DR SUMITAVA DE

2. Beam Modification Definition : Defined as desirable modification in the Spatial distribution of radiation – with in the patient – by insertion of any material in the beam path.

3. Types Of Beam Modifying Devices A. Field blocking and Shaping Device : i . Shielding Block ii. Custom Block iii. Asymmetrical Jaws iv. Multi leaf Collimator B. Compensator C. Wedge Filters D. Beam Flattening Filters E. Bolus

4. Other modifying devices Beam spoiler B . Beam flattening filter C . Breast cone D. Penumbra trimmer E . Electron beam Applicator

5. Shielding To protect the critical structures around the treated volume by using various devices . The aims of shielding are : i ) To protect critical organs. ii) Avoid unnecessary irradiation to surrounding normal t issues . iii)Matching adjacent fields It should have following characteristics : i )High atomic number . ii)High dens ity . iii)Easily available. iv)Inexpensive.

6. Shielding block are most commonly made of lead. Shielding can be two type : Positive : Where central area is blocked . e.g . lung block. Negative : Where peripheral area is block. e.g. Head & Neck.

7. Shielding It’s thickness depends on : Attenuation of shielding material : Half value layer (HVL) is defined as the thickness of an absorber required to attenuate the intensity of beam to half its original value . For practical purposes , the shielding material which reduces beam transmission to 5% or less of its original is considered acceptable. The number of HVL (n) :

8. Custom blocks Material used for custom blocking is known as the wood`s metal( cerrobend ) Commercial names : i ) Lipowitz`s alloy ii) Cerrobend Melting point : 70°C Density : 9.4 g/cm cube at 20° c ( 83% of lead ).

9. The Thickness ratio depend on following factors : a. Compensator to surface distance, d b. Thickness of the missing tissue . c. Field size . d. Depth . e. Beam quality . Types of Compensators : 1. Two-dimensional compensator . 2. Three- dimensional compensator .

10. Wedge filter a. Wedge shaped absorber causes a progressive decrease in intensity across the beam b. Results in tilt of the isodose curve and degree of the tilt depends upon the wedge filter . c. Material: tungsten , brass, lead or steel .

11. Bolus Definition : A tissue equivalent material used to reduce the depth of the maximum dose ( D max ). Used : A bolus can be used in place of a compensator for kilovoltage radition to even out the skin surface contours . In megavoltage radiation bolus is primarily used to bring up the build up zone near the skin in treating superficial lesion .

12. Type of wedges Physical wedges - Individualized wedge - Universal wedge - Motorized wedge Dynamic wedges . Enhanced Dynamic wedges .

13. luminescence. Some materials upon absorption of radiation retain part of absorbed energy in metastable states. This energy is subsequently released in the form of ultraviolet, visible, or infrared light, the phenomenon is called luminescence. Two types of luminescence, ( i ) fluorescence and (ii) phosphorescence, are known, which depend on the time delay between stimulation and emission of light. Fluorescence occurs with time delay of 10−8   s, phosphorescence occurs with time delay of more than 10−8  s or with the suitable excitation with heat or light . Incident ionizing radiation creates the electron hole pair in the crystal structure. The liberated electron is moved (promoted) to the conduction and migrates to the electron trap. At the same time hole migrates (along the valence band) to a hole trap .

14. Energy in the form of heat for TLD or light for OSLD is given to electron and hole to escape from their traps. Finally electron hole pair combines at the luminescent center and releases (emits) light. If the exciting agent is heat, the phenomenon is called the thermoluminescence and the dosimeter is called the thermoluminescent dosimeter (TLD). If the exciting agent is light, the phenomenon is known as optically stimulated luminescence (OSL) and the dosimeter is called as optically stimulated luminescent dosimeter (OSLD ).

15. Thermoluminescent Dosimeter (TLD) Many TLD materials are available, the widely used TLD materials are LiF:Mg , Cu, P, LiF:Mg , Ti, CaSO4:Dy, etc . The elements mentioned after the TLD are the dopants or impurities. The dopants are used to create the metastable states or traps. TLDs are available at various shapes and sizes such as powder, chip, rods, disc, and ribbon depending upon their dosimetric requirement. When TLD is heated, because traps differ in depth, probability of escaping from trap is proportional to temperature. This gives rise to distinct glow peaks Heating (up to certain temperature) a TLD gives a glow curve, which is a graph of intensity as a function of temperature. For reuse of TLD annealing has to be done, by which traps are emptied. For annealing process TLD has to be heated at 400 °C (approximately) for 1 h.