Contention shells are frequently used in radiotherapy. They allow improvements in the reproducibility of treatments and take part in quality assurance. The aim of our survey is to establish how centers use contention masks. Ninety-five percent of the centers questioned use contention masks. The fixation device is in most cases made of Plexiglas. The masks are always made of thermoplastics materials with low modeling temperature. They are made just before simulation or scanner by a radiographer. The surface dose problem can be overcome by two different techniques. One consists in the stretching of the material to reduce the density. The other consists of cutting out the mask where the irradiation fields are projected. Whatever the technique, there is a loss of rigidity of the mask, which means the immobilization is not so effective. Despite drawbacks, contention masks have an important role in the quality insurance in radiotherapy.