Reportage in a radiation oncology service in Switzerland. In the dosimetry service, a dosimetrist (treatment planning technician), chooses the treatment operating mode depending on the final dose determined by the doctor. The dosimetrist outlines the tumor and sensitive organs that must not receive any radiation. She then plans the intensity, duration and method of radiation, fixed or arc, to treat the tumor without damaging other organs. A schwannoma which will be treated using radiosurgery. The final image is obtained by merging images from the dedicated scanner as well as the MRI.