The Dependence of Depth Dose on Focal Skin Distance

Abstract

The dependence of depth dose on focal skin distance has been the subject of a number of investigations. Mayneord and Lamerton (1941) reduced the then available depth dose data from a variety of centres for a variety of focal skin distances to one reference focal skin distance using a suitable conversion factor. The basis for their conversion factor is the inverse square law factor which is given by Mayneord and Lamerton used combinations of F and (F + 1)/2 to reduce the published data to a standard focal skin distance. In the depth dose determinations by the group in Saskatoon (Johns, Fedoruk, Kornelsen, Epp and Darby, 1952; Supplement No. 5), careful measurements were made of the variation with focal skin distance. Their measurements showed that the conversion factor usually lay between (1 + F)/2 and F and varied from F to (1 + F)/2 as the area of the field increased from zero area to a large field. The factor was also shown to be a complicated function of the quality of the radiation. In calculations of tumour dose in rotation therapy, use has been made of the concept of tumour air ratio (Johns, Morrison and Whitmore, 1956; Johns, Whitmore, Watson and Umberg, 1953). This has been defined as the ratio of the dose at the point in tissue to the dose at the same point in space “in air”.