6 Modern Imaging in Brachytherapy

144 Modern Imaging

see Fig 15.10). The D90 (dose that covers 90% of the PTV) and the V100 (Volume that receives the prescribed dose) can also be given (Fig. 5.15, 14.14).

Fig 5.14: CT based 3D planning for intraluminal oesophageal brachyherapy with a 15 mm diameter applicator with delineation of the luminal surface and the outer contour of the oesophageal wall/GTV on transverse CT images. Thickness of the oesophageal wall with the applicator in place is from 3-8 mm. 3D reconstruction of the applicator is shown together with isodose levels in the coronal and sagittal plane. Dose level of 6 Gy is prescribed at 7 mm into the oesophageal wall, resulting in 12 Gy at the luminal surface. Reference dose at 5 mm is 8 Gy. (level 2) In combination treatment with external radiotherapy the respective parameters must be given for both modalities: at the ICRU point for external beam therapy, at specific reference points for brachytherapy. As image matching between brachytherapy and external beam therapy is hardly feasible at present, the respective parameters are usually added taking into account the various biological effects of different dose rates and different doses per fraction.