WHEN A superficial x-ray therapy plant is calibrated using a thimble ionization chamber, it is the free-air output of x-rays that is determined. Such free-air doses of x-rays are unsuitable for dermatologic practice, a fact that will be appreciated by contrasting the effect of delivering 100 r in air to areas of different size. If the treated area is the size of a face, back-scatter of x-rays from the body tissues will increase the dose at the surface of the skin to as much as 135 r, whereas delivery of the same air dose to an area of 1 square centimeter may result in a skin-surface dose as low as 101 r. Obviously, the effects on the skin will vary, even though the air dose delivered was the same in each instance.
In order to obtain constant therapeutic effects, all doses given in dermatologic x-ray treatments should therefore be
LANGLEY RA. SIMPLE SCHEME FOR DERIVING X-RAY SKIN-SURFACE DOSE TIMES. AMA Arch Derm Syphilol. 1953;67(5):484–487. doi:10.1001/archderm.1953.01540050048010
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