6 Modern Imaging in Brachytherapy

Modern Imaging

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Fig 5.5B: Transverse endorectal ultrasound image showing the

dimensions of the prostate in width and thickness and the urethra and the rectum. In addition there is evidence of gross tumour in the right posterior-lateral part of the prostate.

Fig 5.5C: Distribution of iodine-125 ultrasound plane of the prostate volume study. The prostate, the PTV, the urethra, and the rectum are delineated. The number of seeds in the volume and their topographic position within the 3D co- ordinate system can be precisely calculated, determined and displayed in different orientation. Isodose lines are shown in their relation to the PTV, the urethra and the rectum with the prescribe d dose encompassing the PTV (“V100”). Certain dose constraints for the urethra and the rectum are taken into account.

seeds (filled circles) in one

Insert Fig 5C width 8 height 5.8

3.2 Intraluminal Brachytherapy For intraluminal treatment image based brachytherapy may include different procedures depending on the purpose of treatment. In a definitive treatment setting with curative intent , a comprehensive approach is used, as the contribution of intraluminal brachytherapy to local control and cure is essential. Therefore, for the definition of PTV, gross tumour thickness, and length are measured exactly, based on clinical examination, endoscopy, CT/MRI and/or US. Tumour thickness means the radial distance from the luminal surface to the outer tumour border. Accurate measurements and imaging make it possible to precisely define the GTV, the PTV, and the position of the prescription point, and to estimate in how far the prescribed dose will - in ideal geometry - encompass the PTV (Fig 5.6). However, the image based provisional dose planning in these cases only allows for a rough estimate, as the application itself may change the topography considerably (vagina, oesophagus), or as the tumour relation to a small diameter applicator in a large lumen (bronchus) can only be accurately determined during the procedure itself (Fig 5.7).