9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 201

The dose level at point A may - or may not - be equal to the dose level defining the Treated Volume, depending whether the radiation oncologist specifies that the dose at point A defines the Treated Volume (i.e., the dose necessary to achieve the goal of the treatment). The dimensions of the point A volume describe the geometrical features of the pear and banana shape isodoses (long vs short, wide vs small, thick vs thin) depending on the source arrangement, for the same dose to point A and the same TRAK. 8.4.3 Reference volume 8.4.3.1 Definition The reference volume is the volume encompassed by the reference isodose, selected and specified to compare treatments performed in different centres using different techniques. The dose selected to define the reference volume shall imply a consensus at an international level (or at least between the involved centres). 8.4.3.2 Rationale for the concept of reference volume When comparing brachytherapy applications performed in different centres using different techniques, different volumes have to be compared: the Treated Volumes and the point A volumes (they have also to be correlated with the respective PTVs). However, for a given application, Treated Volumes (or point A volumes) of different dimensions may be reported depending on the dose level selected to define these volumes. Inversely, applications reported as delivering the same (nominal) dose may actually be quite different if this dose corresponds to different volumes (24). Therefore, for appropriate comparisons, a dose level must be chosen and agreed as “reference dose”. The volume encompassed by that dose is the “Reference Volume”. The selected reference dose may or may not be equal to the dose at point A, which may, in turn, be equal or not to the dose defining the Treated Volume. 8.4.3.3 Selection of the reference dose level Firstly, as a sine qua non condition, selection of the reference dose level implies a consensus between centres: the goal for introducing the concept of the reference volume is indeed to allow centres to perform relevant comparisons. Secondly, to be clinically relevant, the reference volume should be rather close to the Treated Volume and/or the „point A volume“ . In ICRU Report 38 (43), a reference dose level of 60 Gy is recommended. This dose is indeed appropriate for very early disease (e.g. stage IA) when brachytherapy alone is applied, or in stage I and proximal IIB, when brachytherapy is combined with radical surgery. In such situations, the dose defining the Treated Volume is close to 60 Gy (28).