The invention provides radiotherapy apparatus comprising a source or therapeutic radiation, a detector and a controller. The controller is adapted to receive dose delivery instructions and patient data including patient geometry, and calculates an estimated dose distribution prior to any treatment； it receives the output of the detector, and is adapted to combine this with the patient data to calculate an estimated actual dose distribution； and then compares the predicted dose distribution to the actual dose distribution. The apparatus may issue an alert if the difference between the predicted and actual dose distributions exceeds a predetermined threshold value, or may cause treatment to cease. The predicted dose distribution may be carried out prior to treatment. The analysis may involve back-projection of stored radiation fluence. The patient data may be derived from a CT or MRI scan. The detector may be a portal imager or EPID.
