The present invention relates to the field of radiation therapy. In particular, the invention concerns systems and methods for monitoring intra-fraction motions of patients in connection with treatment cancer in radiation therapy system. A patient marker is attached on the nose of the patient and images of the patient marker and reference markers of a reference tool is captured at predetermined time intervals, wherein the reference tool comprises reference markers and is arranged in a defined position relative to a patient fixation arrangement for fixation of the patient during treatment. A position of the patient marker relative the reference tool is determined based on images captured by an optical tracking system, wherein changes in the position provide information if the patient or a part of the patient has moved.
