Construction of Isodose Lines for 6 MV X Rays by the Decrement-Line Method

Abstract

We have found that the decrement-line method for construction of isodose curves originally applied by Orchard to cobalt-60 gamma rays is satisfactory for x rays generated by a 6 MV linear accelerator. We made “SSD curves,” with 100 cm to the surface and the 100 per cent reference point at the depth of maximal build-up (1.6 cm), and “SAD curves,” with the 100 per cent reference point in the depth at a fixed distance from the target of 100 cm. With a remote locator and a Victoreen ratemeter ionization probe, we determined the locations having decrement values: 0.975, 0.95, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.05, at 4 or 5 depths, for 4 square field sizes in the SAD setup and for 6 fields in the SSD setup. The decrement lines can be regarded as straight lines with sufficient accuracy, and, in fact, decrement measurements would be needed only at two depths: 5 and 15 cm being suitable for the SSD setup, and 7.5 cm above and 7.5 cm below the reference point (axis) for the SAD setup. The decrement line parameters (intercept and slope) are to a good approximation linear functions of field width, so that calculations can be made for any field size on the basis of measurements for a few field sizes. Knowledge of central axis values vs. depth is necessary. To a good approximation this is a single-exponential relationship, except near the surface. The slope µ (effective), is a smooth continuous function of W, the length of the side of the square field. The figure is a comparison of a set of isodoses constructed by use of decrement values (continuous lines) with curves measured directly (dashed curves). The agreement seems satisfactory. The time required to construct isodoses by this method is less than that involved in direct measurement and could be further decreased by use of a digital computer. An additional advantage is that it is not necessary for the accelerator to be available to the physicist after the basic measurements have been made.