6 Modern Imaging in Brachytherapy

128 Modern Imaging

Reliability of the applicator system with the specific material used has to be checked carefully for the respective imaging system with regard to dimensions and topography. Geometrical accuracy is assumed to be almost perfect in CR (projection film imaging), CT and ultrasound. Fluoroscopy only provides sufficient geometrical accuracy around the centre of the screen. Distortions from MRI – depending on field strength and image acquisition protocol – do not play a major role in image assisted brachytherapy, if some basic variables of MRI image distortion are taken into account (8). As there is usually minor distortion around the centre of the magnet, this area should consequently correspond to the centre of the region of interest on the image (PTV) and to the projected applicator. Reproducibility of the topography - as presented on the image - comparing provisional and definitive treatment planning (including quality control) means how far the anticipated and the actual geometry will match. This mainly depends on the applicator position, its impact on the topography of the tumour and the organs at risk and also on tissue changes (for example oedema) taking place after application. In order to achieve a high degree of reproducibility, organ movement and organ filling are the major parameters which have to be carefully controlled. Furthermore, the conditions for image acquisition are to be kept as identical as possible. This can be best achieved when using CR, CT, and MRI and is somewhat more difficult when using ultrasound, due to the variable positioning of the ultrasound device.

A

C

Fig 5.2: Applicator geometry and selection of appropriate image orientation shown for an intracavitary gynaecologic brachytherapy with a ring applicator in place using MRI: the slice orientation is chosen parallel and orthogonal to the axis of the intrauterine tube and the coronal (C) and sagittal (A,B) midplane of the ring: parasagittal (C), paracoronal (A), and paratransverse (B). The cervix cancer (high signal intensity) can be clearly discriminated and the tumour topography can be precisely related to the applicator.

B