A monitoring system for monitoring the positioning of a patient relative to a treatment apparatus operable to deliver radiation along a radiation path, the monitoring system comprising: - a camera system operable to image a surface of the patient being monitored, - a 3D position determination module configured to process images received from the camera system; - a model generation module operable to process data generated by the 3D position determination module and convert said data into a 3D model of the imaged surface of the patient being monitored; - a generated model store configured to store said 3D model of the imaged surface of the patient being monitored; - a target model store operable to store a previously generated 3D model surface of the patient, configured as a target surface; - a comparison module operable to determine a transformation, including rotations and translations, required to match the generated model of the surface of the patient stored in the generated model store with the target model of the target surface stored in the target model store; wherein during treatment said camera system is configured to continuously monitor the patient and update the generated 3D model of the imaged surface of the patient being monitored; wherein the monitoring system furthermore comprises: - an offset determination module operable to: i) utilize the transformation determined by the comparison module; ii) receive data identifying the orientation of the current path along which the treatment apparatus generates a beam of radiation; iii) determine an offset value indicative of the extent and direction of movement of the patient relative to an axis corresponding to the current radiation path for radiation; and wherein iv) the offset value is determined as the modulus of a vector corresponding to the calculated transformation for matching the generated surface with the target surface multiplied by the sine of an angle corresponding to the angle between said vector and the determined current radiation path of radiation generated by the treatment apparatus, wherein v) the offset value is compared with a threshold to determine whether the motion of a patient is sufficient to justify generating a warning or halting off the treatment. - a camera system operable to image a surface of the patient being monitored, - a 3D position determination module configured to process images received from the camera system; - a model generation module operable to process data generated by the 3D position determination module and convert said data into a 3D model of the imaged surface of the patient being monitored; - a generated model store configured to store said 3D model of the imaged surface of the patient being monitored; - a target model store operable to store a previously generated 3D model surface of the patient, configured as a target surface; - a comparison module operable to determine a transformation, including rotations and translations, required to match the generated model of the surface of the patient stored in the generated model store with the target model of the target surface stored in the target model store; wherein - an offset determination module operable to: i) utilize the transformation determined by the comparison module; ii) receive data identifying the orientation of the current path along which the treatment apparatus generates a beam of radiation; iii) determine an offset value indicative of the extent and direction of movement of the patient relative to an axis corresponding to the current radiation path for radiation; and wherein iv) the offset value is determined as the modulus of a vector corresponding to the calculated transformation for matching the generated surface with the target surface multiplied by the sine of an angle corresponding to the angle between said vector and the determined current radiation path of radiation generated by the treatment apparatus, wherein v) the offset value is compared with a threshold to determine whether the motion of a patient is sufficient to justify generating a warning or halting off the treatment. i) utilize the transformation determined by the comparison module; ii) receive data identifying the orientation of the current path along which the treatment apparatus generates a beam of radiation; iii) determine an offset value indicative of the extent and direction of movement of the patient relative to an axis corresponding to the current radiation path for radiation; and wherein iv) the offset value is determined as the modulus of a vector corresponding to the calculated transformation for matching the generated surface with the target surface multiplied by the sine of an angle corresponding to the angle between said vector and the determined current radiation path of radiation generated by the treatment apparatus, wherein v) the offset value is compared with a threshold to determine whether the motion of a patient is sufficient to justify generating a warning or halting off the treatment.