16 Cervix Carcinoma

Cervix Cancer 339

8.6. Common language in intracavitary brachytherapy: ICRU 38 recommendations The existence of many different methods and traditions for prescribing, recording and reporting intracavitary brachytherapy applications (as reviewed in the preceding chapters) implies the need to agree on common language and terminology to facilitate communication between centres and to make possible reliable and relevant comparison of methods and clinical results. 8.6.1 The ICRU 38 recommendations (63). The ICRU recommendations have been described and discussed in detail in chaper 2 on Physics and 6 on Reporting. They are applicable to all types of intracavitary brachytherapy applications; they are not be repeated here. Only some aspects which are specific to cervix brachytherapy are discussed. These recommendations address the following:

™ description of the technique used, ™ total reference air kerma (TRAK), ™ description of the reference volume, ™ absorbed dose at reference points, ™ time dose pattern.

Total reference air kerma (TRAK): The TRAK is a well defined physical quantity which can be easily calculated for any brachytherapy application. The TRAK is an important quantity and all biological effects are directly related to the TRAK. However, this relation is not unique : it depends on the time-dose pattern and geometrical conditions. In order to correlate with the biological effects, the TRAK has first to be weighted by a factor related to the time-dose pattern. The numerical value of this factor depends on the biological system and effect, and on dose level and dose rate. It thus varies with the distance from the sources. In addition, the absorbed dose and the biological effects on the tissues located at a certain distance from the sources can be predicted from the TRAK (provided that a weighting for the time factor has been applied). In contrast, the dose distribution and the effects on the tissues located close to the sources depend on the source arrangement and can thus not be predicted from the TRAK alone. A direct relation between the TRAK and the dose distribution close to the sources is valid only for a given source arrangement (within a given „system“). 60 Gy reference volume and its dimensions (see also Fig 14.16) : In intracavitary applications, the dose gradient is very steep as a function of the distances to the sources, especially in the vicinity of the sources. For reporting and comparison purposes, it is therefore difficult to define a reference point (as in external beam therapy) where the dose can be considered as representative of the dose distribution in a target or organ volume. An alternative approach to compare different intracavitary applications is to agree on a dose level and to report the dimensions of the tissue volume enclosed in that isodose surface. For reporting and comparison purposes, ICRU 38 defines the Reference Volume as the volume encompassed by the 60 Gy isodose surface. It is independent from the individual target volume and the treated volume as selected by the clinician for a specific treatment. It recommends to report the three orthogonal dimensions of the 60 Gy volume: height h, width w, and thickness t. The three dimensions of the 60 Gy volume, and not only their product should be reported. Due to its pear shape, the product of the three dimensions is about twice the computer calculated 60 Gy volume.