The Production of Isodose Curves and the Calculation of Energy Absorption from Standard Depth Dose Data

Abstract

The physicist, faced with a request for a set of isodose charts for each of a number of X-ray fields, has a choice of two main courses, neither of which is entirely satisfactory. The first course, and that usually chosen, is to make measurements within a phantom for the fields concerned and from these measurements construct the isodose curves. Apart from being time consuming and tedious, this course is beset with difficulties. The ionization chambers of many commercial dosemeters are too large for accurate measurements either near the surface or the geometrical edges of the beam, whilst few such dosemeters or, for that matter, few research instruments have “air-equivalent” walls. Even when the chamber is reasonably small and approximately “air-walled,” other difficulties may be met as in the case of the 100 r chamber of the Victoreen condenser dosemeter. One of us (G.J.N.) has shown that when this chamber is in a phantom, some of the scattered radiation is prevented from reaching the ionization chamber by the thick metal stem of the condenser, and that since the effect varies with the conditions of irradiation it is very difficult to assess the correction which should be applied (cf. Miehlnickel and Rajewsky, 1934). However, the effect is too large to be neglected, as it may be as much as 15 per cent. In addition to dosemeter difficulties are those connected with the phantom material.