Beam modification devices are used in radiotherapy to modify the spatial distribution of radiation within the patient. The main types of beam modification are shielding to eliminate dose to some areas, compensation for tissue heterogeneity or beam obliquity, wedge filtration to tilt isodose curves, and flattening filters to modify the natural beam profile. Common beam modification devices include shielding blocks, compensators, wedge filters, and multileaf collimators. The choice of device depends on the treatment site and goals.
11. Shielding 250 KV 4 MV Lesser amount of scattered radiation with megavoltage radiation means that the attenuation produced by shielding is also more. The higher scatter contribution to the overall dose results in lower dosage adjacent to the shielded area in kilovoltage radiation . 100% 50% 5.0 cm Co 60 (1.25 MeV) 7.0 cm 10 MV 6.5 cm 6 MV 6.0 cm 4 MV Required lead thickness Beam energy
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14. Custom blocks Outline of the treatment field being traced on radiograph using a Styrofoam cutting device. Electrically heated wire pivoting around a point (simulating the source) cutting the styrofoam block Cavities in the styrofoam block being used to cast the Cerrobend blocks.
40. Wedge filters - Design This angle is the wedge angle. % DD at each point is calculated and tabulated A B C D E 105 76 43 39 35 Wedge isodose 62 68 67 55 40 Non wedge isodose E D C B A 1.7 1.1 .79 .71 .87 Ratio (W: NW) 1 .66 .42 .38 - Transmission ratio 0 6.5 13.6 15.2 - mm Pb
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42. Wedge filters Compensators may be used to overcome the deficit or a different wedge angle can be used, so that part acts as a compensator. The body contour can be more curved as a result, the isodose curves are not obtained in the manner desired. A small range of hinge angles can be covered by a given wedge angle without producing significant variation(±5%). A different value of wedge angle is required for different beam angles Solution Problem
