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Plastic platforms often are used to support beam-shaping lead blocks during radiation therapy with high-energy x and
rays. To preserve completely the skinsparing effect of high-energy photons, the
platform should be positioned at least 20
cm. above the surface. Measurements with
LiF thermoluminescent phosphor reveal
that the surface dose increases about 30
per cent with the platform 15 cm. above
the surface, 75 per cent with 10 cm. between the patient and platform and 150
per cent when the distance of separation is
reduced to 5 cm. The variation of surface
dose with position of the platform depends
somewhat upon the size of the radiation
field.
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