[Target volume contouring for radiotherapy planning: application of multimodalities imaging]

The development of three-dimensional conformal radiotherapy (3DCRT) and intensity modulated therapy (IMRT) has enabled high dose radiation to be directed to tumors, this however subject to a perfect demarcation of the tumoral volume and the anatomical structures to be saved. At the same time, imaging modalities have improved their ability to demonstrate the location and the extension of tumors. The precise location of the target volume and the prediction of the dose absorbed by the tumor thus become an essential stage of the planning of the treatment. This planning is mostly determined on CT pictures. The technique usually used is a non enhanced CT allowing a location of the tumor by its mass effect mass or by its spontaneous difference of density from the adjacent structures. However, the clear visualization of the tumoral volume remains sometimes difficult for small-sized tumors or those with limited contrast with regard to surrounding tissues. This lack of contrast represents a source of variability for the demarcation of the target volume which has been highlighted in numerous papers. Tumors must be then localized by correlation with pictures made at the diagnosis time by other imaging modalities, essentially the MRI and the PET. This approach answers the concept of multimodality diagnosis which resumes the principle of complementarity of the various techniques. Every technique indeed brings, according to its physical principle, one or several specific information. It is advisable from then to clarify the contribution of every used imaging modality, this according to the pathologies and their localizations.