9 Reporting in Brachytherapy: Dose and Volume Specification

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minimum recommendation, its physical dimensions should be reported in terms of maximum diameters, in mm (or cm) in three orthogonal directions. The CTV must be defined in plain topographic terms and/or according to a code which conforms with the recommendations for the GTV. 2.2.3 GTV and CTV: pure oncological concepts It must be stressed that the GTV and CTV are purely oncological concepts, and are thus independent of the therapeutic approach. In particular, they are not specific to the discipline of radiation therapy. For example, in surgery, a safety margin is taken around the GTV according to clinical judgement, and this implies the use of the same CTV concept as in radiation therapy. In brachytherapy, as in external-beam therapy, volumes to be irradiated are defined, and thus the same concept of CTV is applied. Furthermore, the CTV concept can be applied to other modalities, e.g., regional chemotherapy, hyperthermia, and photocoagulation. The definitions of GTV and CTV in brachytherapy are thus identical to the definitions given for external-beam radiotherapy in ICRU Report 50 (44) and Supplement to Report 50, ICRU Report 62 (47). The aim is to ensure that the prescribed dose is actually absorbed in the whole CTV, taking into consideration the net effect of all the possible variations of position of the CTV relative to the irradiation source. In external beam therapy, the PTV is defined to enable selection of appropriate beam sizes and beam arrangements. In brachytherapy, the PTV is defined to select appropriate source arrangement, positioning and/or movement control. The dose distribution to the PTV has to be considered as representative of the dose distribution to the CTV. 2.3.2 PTV in external beam- and in brachytherapy In external-beam therapy, to ensure that all tissues included in the CTV receive the prescribed dose, one has, in principle, to plan to irradiate a volume geometrically larger than the CTV : the PTV. The additional safety margin results from a number of factors : (1)-the Internal Margin is intended to take into account the expected physiological movements (e.g., respiration) and variations in size, shape, and position (e.g., stomach, bladder, rectum) of the CTV; (2)-the Set-Up Margin is intended to take into account all variations and uncertainties in beam geometry and patient-beam positioning. The situation is quite different in brachytherapy because the source (or source applicator) is, in general, fixed to the target volume. Therefore, in brachytherapy, the PTV is often considered to be identical to the CTV. There are however exceptions. For instance, in intraluminal brachytherapy, a safety margin is added around the CTV to compensate for inaccuracies or uncertainties in the position of the radioactive 2.3 Planning Target Volume (PTV) 2.3.1 Definition The PTV is a geometrical concept used for treatment planning.