Radiotherapeutic physics. VII. An instrument for dosage estimation with fields of any size and shape.
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 p...
THE CALCULATION OF DOSE DISTRIBUTIONS FOR RECTANGULAR X- AND GAMMA-RAY FIELDS.
A method of calculating isodose curves for rectangular fields which is applicable to X rays of HVT 1, 1·5, 2, 2·5, 3 and 4 mm of Cu, 137Cs and 60Co radiation, is described. The method is based on the finding that, for these qualities, all the area scatter data can be fitted to the same curve if suitable scaling factors are applied both to the area and the scatter dose. For a particular quality and depth the axial scatter dose was evaluated graphically for various rectangular fields. From this data a chart was drawn showing lines of equal axial scatter dose in terms of the length of the sides of the field. The central axis scatter doses for other conditions may be obtained by shifting a transparent copy of this chart in relation to the axes and dividing the new values read from the shifted chart by a factor (the shift and the factor being dependent only on the new quality and depth). Scatter doses at non-axial points may also easily be obtained from the shifted chart. The total doses and hence the percenta...
Calculation of and Errors in the Susceptibility
The measurement of the susceptibility requires three measurements, two of which have to be performed with a substance of known susceptibility. The difference of these last two measurements furnishes the proportionality constant of the apparatus, always employing the same volume, while the difference with the unknown substance gives the required susceptibility.