This invention relates to a fixation device for immobilizing a head of a patient relative a treatment unit or medical device during treatment of the head. The fixation device is provided with an interface unit adapted to be fixated in relation to an adaptor unit for connection to the treatment unit, or to a frame for connection to the medical device. Furthermore, the fixation device is further provided with a sliding plate being slidable in relation to said interface unit in an xz-plane of a coordinate system. The system is a Cartesian coordinate system defined by three orthogonal axis having an x-axis extending in the medial-lateral direction of the patient, an y-axis extending in the anterior-posterior direction, and a z-axis extending in the cranial-caudal direction. The fixation device is further provided with a plate locking means for locking said sliding plate in relation to said interface unit, a bite-block for being inserted into a mouth of the patient, a linking means for linking said bite-block with said sliding plate, at least one pivot point arranged such that the bite-block is movable at least in a plane orthogonal to the xz-plane； and a link locking means for locking said at least one pivot point in a desired position in relation to said linking means so as to fixate the bite-block to the interface unit.
