1. A radio therapy device, comprising: a treatment apparatus for applying radiation to a patient, and a patient monitor, the patient monitor including: a 3D position determination module operable to determine measurements indicative of positions of at least part of a patient undergoing radiotherapy; a clock module operable to associate measurements indicative of positions of at least part of a patient determined by the 3D position determination module with timing data; a model generation module operable to (1) use obtained position and timing data for a plurality of breathing cycles obtained from the 3D position determination module and clock module to generate a component analysis model of the variation in position of at least part of a patient during the plurality of breathing cycles, and (2) model a representation of further position and timing data obtained from the 3D position determination module and clock module during a subsequent period of breathing using the component analysis model; and a comparison module operable to calculate a Mahalanobis distance between the modeled representation of the further position and timing data and a mean representation of the generated component analysis model, and compare the calculated Mahalanobis distance with a threshold value, and halt the application of radiation if the comparison between the calculated Mahalanobis distance and the threshold value indicated the breathing cycle is abnormal.