9 Reporting in Brachytherapy: Dose and Volume Specification

174 Reporting in Brachytherapy

6.3 Reference points (dose levels) for reporting interstitial therapy

6.3.1 Mean Central Dose (MCD) In interstitial therapy, the Mean Central Dose is defined as the arithmetic mean of the local minimum doses between sources in the central plane (or in the central planes if there are more than one) (45). In the case of a single-plane implant, the Mean Central Dose is, in the central plane, the arithmetic mean of the doses at mid-distance between each pair of adjacent source lines, taking into account the dose contribution, at that point, from all sources in the pattern (see Fig 6.4A).

Fig 6.8A and B: Evaluation of dose profiles. Three profiles (B) are drawn along two orthogonal directions through a two-plane implant (A) with eight parallel line sources, 10 cm long, 1.8 cm spacing. The profiles are calculated in percentage of the Minimum Target Dose (thick line) along axes XX, YY and Y’Y’ in the central plane. The profile along the axis YY is the most representative to estimate the Mean Central Dose (MCD), which is the mean of the local minimum. The Mean Central Dose is equal to 118% of the peripheral dose (from ICRU Report 58, [45]). In the case of an implant with line sources in more than one plane, the Mean Central Dose is the arithmetic mean of the local minimum doses between each set of three (triangles) or four (squares) adjacent source lines within the source pattern (see Fig 6.4B). For triangles, the minimum dose lies at the intersection of perpendicular bisectors of the sides of the triangles (geometric centre) formed by these source lines. This point is equidistant from all three source lines. For squares, the minimum is at the intersection of the diagonals. In some complex implants, a single central plane may not bisect or even include all the sources. In these cases, a Mean Central Dose based on one plane can be misleading and it is advisable to subdivide the volume and to choose a separate central plane for each subvolume (see Fig 6.7). Three practical methods are acceptable for determining the Mean Central Dose: