The effect of air spaces on absorbed dose was studied, using thermoluminescent dosimeters (TLD). It was determined that, for cobalt-60 gamma rays, distances of 12 cm or more from the scattering materials were adequate to preserve skin sparing. Also, internal body cavities with dimensions greater than 4 cm in the direction of the beam would experience a reduced dose on the distal surface of the cavities.
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John W. Scrimger (1972) studied this question.