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...
A Survey of Depth Dose Data
A survey has been made of published depth dose and backscatter data. The influence of (1) focal-skin distance, (2) quality of radiation expressed as half-value layer in copper, and (3) area of field on percentage depth dose has been investigated. In the light of this analysis published data have been corrected to standard radiation conditions, and mean values obtained. Tables have been drawn up giving the percentage depth dose for qualities of radiation from 1·0 to 5·0 mm. copper half-value layer at depths to 20 cm. for field areas up to 400 cm.2 and focal-skin distances from 40 to 100 cm. A less comprehensive survey has been made of radiations up to 17 mm. copper half-value layer, the results of which are embodied in curves and tables. Mean values of percentage surface backscatter have been derived from the literature for field areas up to 400 cm.2 for qualities up to 14 mm. copper half-value layer. Information is also given whereby one may derive half-value layers in copper from the radiation conditions.
The numerical calculation of the scattering correction in γ ray absorption measurements
The method of applying the scattering correction in accurate γ ray absorption measurements is discussed, and it is shown that the scattering correction only diminishes, as the solid angle subtended by the chamber at the source is reduced, if the absorber diameter is reduced correspondingly. The method of calculation of the scattering correction in cases in which the absorber is placed very close to the chamber is explained, and the result of the calculation for one particular chamber is shown to be in satisfactory agreement with the experimental values. The scattering correction allows the ionization function of chambers to be determined and an (ionization function)/(wave-length) curve is shown for a steel thick-walled high-pressure ionization chamber.