Beam-shaping platforms and the skin-sparing advantage of Co60 radiation.

Plastic platforms often are used to support beam-shaping lead blocks during radiation therapy with high-energy x and γ 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.