The Use of Standard Isodose Distributions with High Energy Radiation Beams—The Accuracy of a Compensator Technique in Correcting for Body Contours

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Abstract
The use of metal compensators mounted at 20 cm from the skin has proved to be a simple and practicable method of correcting for the irregularity of the patient''s contour in high energy radiotherapy. Compensators are made so that the thickness of aluminium parallel to the direction of the central ray is equal to 0.88 h/[rho], where h is the corresponding thickness of unit density material replaced and [rho] is the density of the aluminium. To avoid errors resulting from the geometry of the system, the density of the compensator material should have the optimum value given by the expression: [rho]=0.88 b/a, where b is the source-skin distance of the particular unit and a is the distance from the source at which the compensator is mounted. The additional errors of compensation introduced by the use of Al instead of a material with an ideal density do not exceed [plus or minus] 4% for the vast majority of cases met with in clinical practice. For a Co unit with a source 2 cm in diameter, compensators may be built up from a stock of aluminium columns. The effect of beam divergence cannot be ignored for units working at 60 or 80 cm source-skin distance, with compensators mounted 16 to 20 cm from the skin, without introducing appreciable errors for steeply sloping contours. The compensator should be scaled down to allow for its position in the beam.
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