9 Reporting in Brachytherapy: Dose and Volume Specification

160 Reporting in Brachytherapy

A GTV may be confined to only part of an organ (e.g . T2a in prostate cancer), or involve a whole organ (e.g., T2b in prostate cancer, IB2 in cervix cancer). The GTV may or may not extend outside the normal borders of the organ tissue involved. For reporting, the GTV should be described in standard topographical or anatomical terms, e.g.“18 mm x 12 mm x 20 mm tumour in the left lobe of the prostate adjacent to but not reaching the capsule“, cT2a. In many situations, a verbal description might be too cumbersome and, therefore, for the purpose of data recording and analysis, a classification system is needed. Several systems have been proposed for coding the anatomical description, some of them are mentioned in ICRU Report 50 (44). Careful identification of the GTV is as important in brachytherapy as in external beam therapy, for at least three reasons: (1) accurate description of the GTV is needed for staging (e.g., TNM), (2) identification of the GTV is necessary to permit recording of tumor response in relation to dose and other relevant factors, which may be used (carefully) as a prognostic factor. (3) an adequate dose must be delivered to all parts of the GTV to obtain local tumour control in radical treatments. 2.2 Clinical Target Volume (CTV) 2.2.1 Definition The CTV is the volume which contains the „gross“ and „subclinical“ disease. Clinically, it thus contains the GTV and a „safety margin” around the GTV (CTV-T) to take into account (probable) subclinical involvement. The CTV may also include other anatomical areas, e.g., regional lymph nodes (CTV-N) or other tissues with suspected (or proven) subclinical involvement (CTV-M). “Subclinical involvement“ may consist of individual malignant cells, small cell clusters, or micro- extensions, which cannot be detected during staging procedures by the methods mentioned above. The cell density is high in the GTV (typically 10 6 /mm 3 ); it decreases in the safety margin from the edge of the GTV towards the periphery of the CTV. The different parts of the CTV thus have to be treated at adequate dose levels (and time-dose patterns) to achieve the aim of therapy, either cure or palliation. If the GTV has been removed by radical surgery, and radiotherapy is needed to residual tissue close to the site of the removed GTV, this volume is also usually designated as CTV-T (e.g., in breast- conserving procedures). Delineation of a CTV requires consideration of factors such as the local invasive capacity of the tumor and its potential to spread, for example, to regional lymph nodes (based e.g. on histology). The definition of CTV boundaries requires a clinical decision (31,62). In some cases, this decision is based on probability evaluation (when data are available), but it often implies an arbitrary choice (e.g., endovascular brachytherapy (86)). The final decision rests on the clinical experience and judgement of the radiation oncologist. 2.2.2 Recommendations for reporting The CTV (like the GTV) is a purely clinical-anatomical concept : it should therefore be described in terms of the patient’s anatomy and the tumor extent, independently of any dose distribution. As a