Wax retraction as a technique for compensating the effect of surface irregularities in high-energy radiotherapy.

Abstract

The technique of filling an irregular body contour with tissue-equivalent wax and retracting the wax in the beam direction, without making an allowance for beam divergence, has been examined from the point of view of (a) electron contamination on the skin; (b) errors in depth dose which arise from loss of scatter and also from any change in the path of the primary ray. Skin contamination at a given point has been found to depend on the angle subtended there by the irradiated wax. This leads to the simple working rule that if a retracting distance equal to the average field dimension is chosen, the skin dose will be reduced to 50 per cent of its value before retraction. Depth dose errors, i.e., deviations from the values as read in the normal isodose chart, have been found to depend chiefly on retracting distance and the thickness of the retracted wax. The technique is feasible provided a retracting distance can be found which gives an acceptable skin dose and small depth dose errors. These may be estimated from the appropriate graphs. The conditions are most easily fulfilled for small fields.