The invention involves monitoring patient movement, in particular respiratory movement, of a patient (30) positioned on a couch (20) in connection with a radiation gantry (10). A 2D or 3D pattern (115) is projected onto the patient (30). At least a first portion of the pattern (115) is detected by a first detector (120) arranged on a first side of the gantry (10). At least a second portion of the pattern (115) is detected by a second detector (130) arranged on a second opposite side of the gantry (10). Pattern detection data is generated by the two detectors (120, 130) and is used for generating information representative of movement of the patient (30) on the couch (20). The information can be used, for instance, for prospective and/or retrospective gating.
