9 Reporting in Brachytherapy: Dose and Volume Specification

176 Reporting in Brachytherapy

3. Inspection of dose distribution: the dose distribution is plotted in the central plane. With isodose lines varying by 5% (at most 10%) of the local dose in the central region, the local minima can be determined by inspection. The mean of these local minima is the Mean Central Dose (Fig 6.10). This method is often preferred for complex implants with line sources. 6.3.2 Minimum Target Dose The Minimum Target Dose is the dose selected and specified by the radiation oncologist as adequate to treat the PTV. It corresponds to the prescribed dose in many instances. It is related to the source arrangement and is the dose delivered at the periphery of the PTV. The application is planned in such a way that all points of the PTV receive a dose (at least) equal to the Minimum Target Dose. The Minimum Target Dose is known in some American centers as the ‘minimum peripheral dose‘ (19). It is equal to about 90% of the prescribed dose in the Manchester System for interstitial therapy. It is known as the ‘reference dose‘ in the Paris System, where it is equal to 85% of the Mean Central Dose (MCD). 6.4 Volumes for reporting in interstitial therapy 6.4.1 Treated Volume As defined in section 2.4, the Treated Volume is the tissue volume that, based on the actual implant, receives at least a dose selected and specified by the radiation oncologist as appropriate to achieve the purpose of the treatment (e.g., tumour eradication or palliation). Following the definition of the Minimum Target Dose (see above), the Treated Volume is encompassed by an isodose surface, the value of which is the Minimum Target Dose. The Treated Volume should, in principle, entirely encompass the CTV (however, this may not necessarily always be the case). 6.4.2 High-dose regions In order to correlate radiation dose with late damage, the high-dose regions around sources should be assessed (Fig 6.5 and 6.11)

Fig 6.11A and B: Tongue implant, using five loops of 8 cm iridium wires with activity of 68 MBq cm-1 (A) Radiographs of the implant. (B) Dose distribution in the central plane of the implant (From Wambersie and Battermann [115]).