9 Reporting in Brachytherapy: Dose and Volume Specification

162 Reporting in Brachytherapy

source(s) relative to the patient’s organ. Such inaccuracies can be due to displacement of the source (in the longitudinal or radial direction), patient motion, etc. The width of this safety margin should, ideally, be based on systematic evaluation of the uncertainties. It may differ according to the organ being treated, the clinical target volume and also the type of applicator and technical devices used. With some techniques, there are uncertainties about consistency of source position (moving sources, fractionated techniques) or alteration of source or applicator position (intracavitary applications, permanent implants) during the application. Remarks: 1°) In external therapy, the two steps (localisation of CTV and treatment planning) can always be dissociated and therefore checked separately. However, in interstitial therapy, a better evaluation of the tumour extent (and regression after external beam therapy) may often be obtained by the clinician at the time of application (due e.g. to general anaesthesia). The final decision on the CTV may then be modified. 2°) Due to the high density of the cancer cells in the GTV (about 10 6 /mm 3 ), a high irradiation dose must always be delivered to all parts of the GTV in radical treatments. 3°) A CTV may be treated by 2 (or more) PTVs (e.g., external and brachytherapy). In particular, because the cancer cell density is higher in the GTV compared to tissues with only subclinical disease, different dose levels may be prescribed and thus several PTVs be identified. This is the case, for example, in ‘boost‘ therapy where the ‘higher-dose‘ volume (often containing the GTV) is located inside the ‘lower-dose‘ volume. For example, 50 Gy may be prescribed to a large PTV followed by an additional 35 Gy to a smaller PTV („boost“) corresponding to the GTV only. These two PTVs may be referred to as PTV-50 and PTV-85, respectively. Another example is the treatment of cervix cancer where the central part of the PTV is mainly treated with high-dose brachytherapy and the lateral extensions by external beam and a lower dose contribution from brachytherapy. 2.4 Treated Volume The treated volume is the volume of tissue which, according to the implant as actually achieved, receives a dose at least equal to the dose selected and specified by the radiation oncologist as being appropriate to achieve the goal of the treatment. The Treated Volume is thus encompassed by an isodose surface corresponding to that dose level, which is the Minimum Target Dose (section 6.3.2, 7.4.3). This isodose surface should ideally match the PTV as closely as possible, it should entirely encompass the CTV/PTV, but may be larger depending on the available sources and source arrangement. The Treated Volume (and the PTV) thus depends on the irradiation technique.

2.5 Irradiated Volume

The Irradiated Volume is the tissue volume, larger than the Treated Volume, which receives a dose considered to be significant in relation to normal tissue tolerance.