BEAM MODIFYING DEVICES IN RADIOTHERAPY
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This document discusses various types of beam modification techniques used in radiation therapy, including shielding, wedges, compensators, and flattening filters. It describes the materials and properties used for each technique, how they modify the beam and dose distribution, and their purposes such as shielding organs at risk, evening skin dose, or accounting for tissue heterogeneity. The conclusion emphasizes that beam modification can increase dose conformity to the target while sparing normal tissues, but requires careful planning and treatment.
BEAM MODIFYING DEVICES IN RADIOTHERAPY
- 2. SHIELDING COMPENSATION WEDGE FILTRATION FLATTENING TYPES OF BEAM MODIFICATION
- 3. • Commonly used shielding material for photon – Lead • Thickness depends – Energy of radiation • Kv range – Sheets of lead placed directly on the surface • Megavoltage radiation – Thicker blocks (Placed up in shallow trays) Choice of shielding material • High Atomic Number • High density • Easily available • Less costly Properties of ideal shielding material SHIELDING
- 4. Metal used-LIPOWITZ or cerrobend Meltingpoint-70°c Thickness -7.5 cm Types Positive blocks --------- central area blocked Negative blocks--------- peripheral area blocked Diverging blocks Follows geometric divergence of beam Minimizes block transmission penumbra Custom blocks
- 5. Bank of large no:of collimating blocks or leaves Can be moved automatically Independent of each other to generate a field of any shape 40 or more pairs of leaves having width 1cm or less Thickness 6 - 7.5cm Made of tungsten alloy MULTI LEAF COLLIMATORS
- 6. Are beam modifying devices which evens out the skin surface contours, while retaining the skin sparing advantage Allows normal depth dose data to be used for much irregular surfaces Also used for: Compensate for tissue heterogeneity Used in total body irradiation Compensate for dose irregularities arising due to reduced scatter near field edges (e.g.;- Mantle fields)and horns in the beam COMPENSATORS
- 7. Special beam modification device where shadow trays made of Lucite are kept at a distance from skin Based on the principle that relative surface dose increases when surface to tray distance is reduced First used by DOPPKE to increase dose to superficial neck nodes in head & neck cancers using 10MVphoton beams BEAM SPOILERS
- 8. A beam modifying device, which causes a progressive decrease in intensity across the beam,resulting in tilting the isodose curves from their normal positions. Degree of the tilt depends up on the slope of the wedge filter Material:tungsten,brass,lead,or steel Usually wedges are mounted at a distance of 15cm from the skin surface The sloping surface is made either straight or sigmoid in shape A sigmoid shape produces a straighter isodose curve WEDGE FILTERS
- 9. TYPES OF WEDGES Individualized wedge Universal wedge Dynamic wedges Virtual wedges Pseudo wedges
- 10. A beam flattening filter reduces the central exposure rate relative to that near the edge of the beam Used for linear accelerators Due to the lower scatter the isodose curves exhibit “forward peaking” The filter is designed so that the thickest part is in the centre Material: copper or brass FLATTENING FILTERS
- 11. Tissue equalent material Used to reduce depth of (Dmax) Can be used in place of a compensator for kilovotage radiation to even out the skin surface contours In mega voltage radiation bolus is primarily used to bring up the buildup zone near the skin in treating superficial lesions the thickness of the bolus used varies according to the energy of the radiation In megavoltage radiation: 6mv: 7-8mm 10mv: 12-14mm 25mv: 18-20mm Co60: 2-3mm BOLUS
- 12. A beam modifying and directing device used for tangential fields therapy Advantages: Directs beam to the central axis of the area of interest,where a tangential beam is applied to a curved surface Helps position, the patient with an accurate SSD End plate provides compensation, enhances surface dose and presses down the tissue Effective shielding of lungs BREAST CONE
- 13. Consists of extensible, heavy metal bars to attenuate the beam in the penumbra region Increase the source to diaphragm distance redusing the geometric penumbra. Another method is to use secondary blocks placed close to the patient (15-20cms) PENUMBRA TRIMMERS
- 14. Electron field shaping is done using lead cutouts. For a low-energy electrons(<10 MeV),sheets of lead, less than 6mm thickness are used The lead sheet can be placed directly on the skin surface Shields can also be supported at the end of the treatment ELECTRON BEAMS
- 15. A device to widen the thin pencil beam (3mm) of electrons Metallic plates of tin, lead or aluminum are used Disadvantages: Beam attenuation Generation of bremsstrahlung radiation Advantages: Less prone to mechanical errors Less expensive Requires less instrumentation Now a days dual foil systems are used, which compare well with scanning beam systems SCATTERING FOIL
- 16. Beam modification increases conformity allowing a higher dose delivery to the target ,while sparing more of normal tissue simultaneously Megavoltage radiotherapy is better suited for most forms of beam modification due to its favorable scatter profile. However any beam modification necessitates a close scrutiny on every phase of the planning and treatment process CONCLUSION