A monitoring system for monitoring the positioning of a patient (20) relative to a treatment apparatus (16) operable to deliver radiation along a radiation path is provided. A model generation module (58) is arranged to generate a model of a surface of a patient (20) being monitored. A comparison module (64) determines a transformation required to match the generated model to the stored model target surface. An offset determination module (68) then utilizes the transformation and data indicative of a current position of a treatment apparatus (16) to determine an offset value indicative of the offset of the surface of a patient (20) relative to an axis corresponding to a determined current radiation path for radiation generated by the treatment apparatus (16) applied to a patient (20). If the offset value exceeds a threshold, a warning may be generated or treatment may be halted.
