1. A method of detecting a delivery error in a radiation delivery device, the method comprising using a computer to carry out the steps of: generating a treatment plan for a patient, the treatment plan including an intended fluence; delivering radiation to the patient according to the treatment plan; acquiring output fluence information from the radiation delivery device after delivery of the treatment plan; comparing the output fluence information and the intended fluence to determine a fluence variance; and applying a dose calculation algorithm to the fluence variance to generate a dose variance map.