9 Reporting in Brachytherapy: Dose and Volume Specification

178 Reporting in Brachytherapy

Fig. 6.12 (continued): 0.72. By this approach the technique could finally be characterised as intensive treatment in terms of dose and volume compared to commonly reported practice in literature. A similar conclusion was reached for the consecutive HDR brachytherapy experience with a Minimum Target Dose of 10 Gy and a “85% of Mean Central Dose” of 13 Gy . (From Resch, Pötter, Van Limbergen et al. [92]) At present, there is no general agreement on how to relate reference volume and Mean Central Dose. For example, in the Paris System, the reference dose is 85% of the Mean Central Dose. In using a reference volume for reporting, the relation of the reference dose to the Mean Central Dose should always be given (e.g. 90%, 80%, 75%). A relevant comparison must use the same relationship between the Mean Central Dose and the dimension of the reference volume. If this is done, it is possible to compare dose and volume between different treatments for a fixed relation between the reference isodose and the Mean Central Dose. If this is not done, an apparently similar prescribed dose and volume may correspond to totally different implants (Fig 6.12). The reference isodose level for reporting should be that commonly used in literature reports (e.g. 85%). 6.5 Dose uniformity parameters Several indices quantifying the homogeneity of the dose distributions have been proposed (see, for example (74,96,118). Two parameters describing dose uniformity for interstitial implants are recommended in ICRU Report 58 (45). They can be derived directly from the concepts of Minimum Target Dose and Mean Central Dose: 1. The spread in the individual minimum doses used to calculate the Mean Central Dose in the central plane expressed as a percentage of the Mean Central Dose; 2. The dose homogeneity index , defined as the ratio of Minimum Target Dose to the Mean Central Dose. 6.6 Additional representation of the dose distribution To appreciate fully the dose distribution of an implant, the use of volume-dose calculations has been advocated (13,63,70). For this purpose, the CTV (or a larger volume including an additional margin) is divided into subvolumes (e.g., voxels) and the dose rate calculated at the centre of each subvolume. The volume receiving at least a specified dose is then defined as the sum of all subvolumes where at the centre at least that dose is received.