A generalized method of rapid dosage estimation with particular reference to 200 KV therapy.

Abstract

A very rapid method of dosage estimation is described which is useful throughout the whole range of so-called “200 kV” therapy. The method relies on the analysis of the Mayneord-Lamerton survey data into primary and scatter contributions. For points not on the central axis the scatter contribution is a function of the position in the field. By suitable choice of coordinates it is shown that the ratio of the dose due to scatter at a given point can be easily related to the central axis scatter. The ratios were determined experimentally by means of an optical integrating machine previously described, and are tabulated for fields of simple shape, including circles, squares, rectangles, and some triangles. The consistency of the method is examined and one of the examples is chosen so that it may be checked against recently published direct measurements and against the result of calculations using Meredith and Neary's method. It is also shown how many fields of complex shape, such as are produced by adjustable diaphragm devices or by partial masking of regular fields, can be treated by taking them as combinations of fields already discussed.