9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 179

Fig 6.14: Example of a Cumulative Dose- Volume Histogram (GDVH) of the CTV for a prostate implant. The dose (in % of the prescription dose, PD) is given on the horizontal axis. The volume (in % of the CTV) encompassed by different isodose values is given on the vertical axis. The minimum peripheral dose (or Minimum Target Dose, see 6.3.2) is the highest dose still encompassing the CTV. In this example, about 10 % of the CTV does not receive the prescribed dose. (From Van der Laarse and Ruthmann [105]).

Fig 6.13 : Natural Dose-Volume Histograph (NDVH) of the tongue implant shown in figure 6.11. Treatment dose rate of 1 Gy h -1 was chosen to deliver 60 Gy in 60 h. (From Anderson [6]).

Because of high dose gradients, significant differences in calculated volumes may be observed, depending upon the size of the elementary subvolumes. The size of the grid and of the elementary subvolumes used in dose and volume calculations should be clearly stated. Volume-dose data can be represented by means of histograms showing the distribution of fractions of the CTV receiving doses within chosen invervals, especially the Natural Dose-Volume Histogram (NDVH) as published by Anderson (5,6) (Fig 13). With this model, even small differences between implants can be shown. The main characteristic of the NVDH is the peak that occurs with a regular implant of several sources. In fact, the peak dose reflects the basal dose of the Paris System (106). If the implant is less uniform, the peak is wider. So, the NVDH can be used for intercomparison between planned and actual source arrangements (65,106). An example of a Cumulative Dose- Volume Histogram (CDVH) is presented in Fig 14 (105). The value of these alternative representations of the dose distribution as a possible prognostic factor for treatment outcome has still to be established in clinical research.