VII. An Instrument for Dosage Estimation with Fields of Any Size and Shape

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Abstract
In the preceding paper in this symposium the need was stressed for a system of dosage estimation with beamed radiations of more general application than those at present use, and in particular one that could be used for the routine estimation of dose with fields of irregular shape. Brief mention was made of a machine which could fulfil this function, and the purpose of the present paper is to describe more fully the construction and mode of operation of this device. The basic principle of the device is one which has been discussed by various workers—that the analysis of the way in which the dose of scattered radiation at given points on the central axis of a circular field varies with field diameter can be made the basis of the estimation of dose at any point of the volume irradiated by an irregularly shaped field. The essential procedure in this estimation has in the past been one of integration, graphical or otherwise, of a rather cumbersome nature and not well fitted for routine work. However, in the paper of Lamerton and Winsborough (1950) a method was suggested for facilitating this integration, and that method is the basis of the present machine in which the integration is carried out automatically. The actual operation of the machine is very simple. It records the dose (or percentage depth dose) at any point in a field represented by an aperture in a black paper mask moved over a transparent table.

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