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