09 - Treatment Planning III - field shaping skin dose

09 - Treatment Planning III - field shaping skin dose - 1...

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Unformatted text preview: 1 Treatment Planning III: Field Shaping, Skin Dose, Field Separation Tarun K. Podder, PhD, DABR Department of Radiation Oncology Brody School of Medicine Leo Jenkins Cancer Center East Carolina University February 08, 2011 Lecture # 9 (Ch. 13) 2 Some Considerations • Shielding vital organs within radiation field • Minimizing dose to normal tissue • Skin Sparing • Matching of adjacent fields – hot spots, cold spots 3 Field Shaping • Custom Blocking • Independent Jaws • Multileaf Collimators (MLCs) Why is the field shaping required? Methods: To minimize dose to the normal tissue outside the target volume. 4 Field Blocks • Block Thickness 5% transmission of primary beam through the block is considered acceptable. If n is the number of HVL needed to achieve this transmission, 1/2 n = 0.05 n = log 20 / log 2 = 4.32 (between 4.5 to 5 HVL) e.g., HVL Pb (ρ=11.36 g/cc)~1.4cm for 6MV photon  Pb thickness = 6.5cm For MV beams Cerrrobend (ρ=9.4 g/cc) thickness ~7.5cm is commonly used. Cerrobend (70 C melting temp.) contains 50% bismuth, 26.7% lead, 13.3% tin and 10% cadmium. 5 Field Blocks • Block Divergence A: Cerrobend blocks for lung shielding. B: Custom blocks for head and neck. Block cutter. Partial transmission of the beam at the block edge increases transmission pemumbra. Divergent blocks are most suitable for beams having small focal spots. 6 Field Blocks • Independent Jaw vs. Custom Block Comparison of isodose distribution with half the beam blocked by an independent jaw versus a block on a tray. Notice close agreement as well as the tilt of the isodose curves toward the blocked edge. Independent Jaws: • Rectangular blocking • Matching fields or beam splitting – blocked off at central axis; removed divergence • Both reduced dose toward the blocked edge – reduced scattering; hence there is a tilt of the isodose curves. 7...
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This note was uploaded on 01/21/2012 for the course PHYS 6720 taught by Professor Hu during the Spring '10 term at East Carolina University .

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09 - Treatment Planning III - field shaping skin dose - 1...

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