Radiotherapy apparatus comprising; a source of therapeutic radiation for delivering a dose to a patient, an electronic portal image detector for detecting the therapeutic radiation, after it has passed through the patient and after attenuation by the patient, a control unit adapted to: i. receive a sequence of dose delivery instructions; ii. receive data describing the patient and characterising at least the patient geometry; iii. using the dose delivery instructions and the patient data, perform a first computational analysis to ascertain an expected dose distribution to be delivered to the patient by performance of an initial subset of the dose delivery instructions in order to produce a first dose distribution and a first profile indicative of the development of the first dose distribution during its delivery; iv. subsequent to completion of the first computational analysis, cause the apparatus to perform the dose delivery instructions; and v. during the performance of the dose delivery instructions, a) acquire the output of the detector; b) perform a second computational analysis based on the acquired output in conjunction with the patient data to produce an estimated actual subsequent dose distribution and a subsequent profile indicative of the development of the subsequent dose distribution during its delivery, and c) compare the estimated actual dose distribution to the expected dose distribution by comparing the first and the subsequent profiles to ensure that the subsequent dose distribution is developing in the same manner as the first dose distribution. a source of therapeutic radiation for delivering a dose to a patient, an electronic portal image detector for detecting the therapeutic radiation, after it has passed through the patient and after attenuation by the patient, a control unit adapted to: i. receive a sequence of dose delivery instructions; ii. receive data describing the patient and characterising at least the patient geometry; iii. using the dose delivery instructions and the patient data, perform a first computational analysis to ascertain an expected dose distribution to be delivered to the patient by performance of an initial subset of the dose delivery instructions in order to produce a first dose distribution and a first profile indicative of the development of the first dose distribution during its delivery; iv. subsequent to completion of the first computational analysis, cause the apparatus to perform the dose delivery instructions; and v. during the performance of the dose delivery instructions, a) acquire the output of the detector; b) perform a second computational analysis based on the acquired output in conjunction with the patient data to produce an estimated actual subsequent dose distribution and a subsequent profile indicative of the development of the subsequent dose distribution during its delivery, and c) compare the estimated actual dose distribution to the expected dose distribution by comparing the first and the subsequent profiles to ensure that the subsequent dose distribution is developing in the same manner as the first dose distribution. i. receive a sequence of dose delivery instructions; ii. receive data describing the patient and characterising at least the patient geometry; iii. using the dose delivery instructions and the patient data, perform a first computational analysis to ascertain an expected dose distribution to be delivered to the patient by performance of an initial subset of the dose delivery instructions in order to produce a first dose distribution and a first profile indicative of the development of the first dose distribution during its delivery; iv. subsequent to completion of the first computational analysis, cause the apparatus to perform the dose delivery instructions; and v. during the performance of the dose delivery instructions, a) acquire the output of the detector; b) perform a second computational analysis based on the acquired output in conjunction with the patient data to produce an estimated actual subsequent dose distribution and a subsequent profile indicative of the development of the subsequent dose distribution during its delivery, and c) compare the estimated actual dose distribution to the expected dose distribution by comparing the first and the subsequent profiles to ensure that the subsequent dose distribution is developing in the same manner as the first dose distribution. a) acquire the output of the detector; b) perform a second computational analysis based on the acquired output in conjunction with the patient data to produce an estimated actual subsequent dose distribution and a subsequent profile indicative of the development of the subsequent dose distribution during its delivery, and c) compare the estimated actual dose distribution to the expected dose distribution by comparing the first and the subsequent profiles to ensure that the subsequent dose distribution is developing in the same manner as the first dose distribution.