6 Modern Imaging in Brachytherapy

Modern Imaging

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In small tumours with a thickness of only a few millimetres, provisional dose planning will, in principal, reveal that the therapeutic isodose can encompass the PTV (e.g. the whole wall), if – in ideal geometry - the prescription point can be adequately chosen (e.g. at 5-7 mm from the applicator surface). If the tumour thickness related to the luminal surface is significantly thicker than 5 mm, an adequate coverage of this PTV is only possible with a large diameter applicator allowing in addition some overdosage to the mucosa/submucosa, which may be critical. In image based planning of such application, as much as possible of the overdosage volume - receiving more than 200% of the prescribed dose – should be within the applicator itself. This relation is mainly dependent on the diameter of the applicator, its topographical relation to the wall and to the PTV and the prescription point (see chapter on oesophagus). If for example a large thick tumour as shown by imaging (e.g. >10mm) is to be treated, and if intraluminal brachytherapy is regarded as an essential treatment option, it is appropriate to start with external beam therapy. In such case a significant dose of intraluminal brachytherapy applied to the PTV within a boost treatment is only possible after significant tumour shrinkage. In such combination treatments, the respective examinations have to be repeated at the time of brachytherapy in order to be able to delineate precisely the actual GTV at this point, the PTV and the lumen dimensions and to base the technique of application and the dosimetry on these findings.

Fig 5.6: Recurrence at the stump of the left upper lobe bronchus. A: Endoscopic image

B: Drawing in the “bronchial tree” showing the accurate topographic relations and the dimensions related to the bronchus lumen: 18 mm in length and 7 mm in thickness. Curative treatment with brachytherapy alone.