A monitoring system for monitoring the positioning of a patient relative to a treatment apparatus that delivers radiation along a radiation path. The monitoring system comprising: a model generation module 58 to generate a model of a surface of the patient a target model store 62 storing a model of a target surface a comparison module 64 to determine a transformation required to match the generated patient surface model with the stored target surface model and an offset determination module 68 that uses the transformation and data indicative of a current position of a treatment apparatus, to determine if a patient has moved out of position in a manner which is likely to be detrimental to the patient. The monitoring system may monitor the position and orientation of the treatment apparatus and determine the current radiation path generated by the treatment apparatus. Offset determination module 68 may determine an offset value between the centroid of the generated model surface and the target surface in a direction normal to an axis corresponding to the current radiation path. Offset determination module 68 may determine an offset value indicating a degree of overlap between a target region and a generated radiation beam.
