A simple procedure for estimating off-axis doses in a medium irradiated by cobalt-60 beams.

Abstract

In 1964 two interesting papers on the characteristics of dose distributions in cobalt-60 irradiated medium were published, one by Sterling et al. (1) and the other by Orchard (2). Both authors derived their findings empirically from experimental data, Sterling using mathematical equations and Orchard, graphical presentations. Both related the dose at any point in a medium to the dose at a point at the same depth on the central axis of the beam, a relationship we shall call the “off-axis dose ratio.” The original purpose of our investigation was to compare the Sterling and Orchard findings, using measured dose data with our Picker cobalt-60 unit, but in the process of the evaluation we derived a method for determining the percentage depth dose at any point with a simple table which can be used as a supplement to the standard centralaxis depth-dose table. Measurements Orchard found that in the principal plane of the irradiated medium the points with the same off-axis dose ratio at different depths are on a straight line, which he calls a “decrement line.” To test this finding, we set up an experiment to measure the relative doses along transverse lines, perpendicular to the central axis of the beam at several depths ranging from 5 to 20 cm in a cobalt-60 irradiated water phantom using a source-surface distance of 50 cm. The source size of our cobalt-60 unit is 1 cm in diameter, and the distance from the source to the distal end of the collimator is 27 cm. For the measurements we used a small ionization chamber of an internal diameter of 3 mm, moved by a remote-control servo mechanism. At any one depth the maximum reading on the electrometer was first found along the transverse line, and then paired points on each side of the central axis were located, at which the readings were 0.95, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, and 0.1 of the maximum. The midline of the paired points with the same off-axis dose ratio was then taken as the central axis of the beam. Figure 1 shows a typical transverse dose profile at 5 cm depth under the surface, which gives the off-axis dose ratios at various distances from the central axis for field sizes 4 × 4 cm to 15 × 15 cm. Figure 2 is a graph of the same data as in Figure 1, demonstrating the relationship between the distances from the central axis to the 0.95 ..... 0.1 off-axis dose ratio points and the field widths. It can be seen that with the exception of those for ratios 0.95, 0.9, and 0.1, this relationship for all other off-axis dose ratios is linear, indicating that for points with off-axis dose ratios between 0.8 and 0.2 at 5 cm depths the distance of a particular off-axis dose ratio from the central axis increases in direct proportion to the field width. This relationship is also true for other depths.