The Calculation of Dosage Rates from Directional Caliper Measurements

Abstract

The directional caliper, model 2, designed by Green,1 for use in X-ray therapy, like model 1, for use with a teleradium unit,2 if used for dosage rate measurements, gives the position of the point at which the dosage rate is to be measured in terms of the distances from the surface, or skin line, and from the axis of the beam of radiation, this latter forming the third side of a triangle of which two sides and the included angle are given by the caliper readings. Read,3 in connection with model 1, has described an apparatus which, by reconstructing this triangle, enables the dosage rate to be read directly from the isodose diagram of an axial section of the beam, and which can also be used for model 2, although its circular scale arm should be made equal in length to the fixed arm of the caliper. It is proposed to describe here an alternative nomographic method, applicable to both models 1 and 2, and requiring no special apparatus. Let O (diagram 1) represent the centre of the beam, OR the fixed arm of the X-ray caliper, of length r cm. (r = 40), and RP the movable arm with a vertical reading of h cm. that is of total length (r + h) cm., then OP, the distance of the point measured from the axis, is given by:—