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 is captured at predetermined time intervals. The reference markers are arranged in defined positions relative to a patient fixation arrangement for fixation of the patient during treatment. A position of the patient marker relative the reference markers 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.
