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. cause the apparatus to perform the dose delivery instructions and acquire information from the detector as to a delivered radiation fluence; iv. after commencement of the sequence of instructions, perform a first computational analysis based on the acquired information as to the delivered radiation fluence in conjunction with the patient data in order to produce a first dose distribution and a first profile indicative of the development of the first dose distribution during delivery; v. subsequent to completion of the first computational analysis, cause the apparatus to perform the dose delivery instructions a further time; and vi. during the further 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 a subsequent dose distribution and a subsequent profile indicative of the development of the subsequent dose distribution during its delivery, and c) compare the subsequent dose distribution to the first 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. cause the apparatus to perform the dose delivery instructions and acquire information from the detector as to a delivered radiation fluence; iv. after commencement of the sequence of instructions, perform a first computational analysis based on the acquired information as to the delivered radiation fluence in conjunction with the patient data in order to produce a first dose distribution and a first profile indicative of the development of the first dose distribution during delivery; v. subsequent to completion of the first computational analysis, cause the apparatus to perform the dose delivery instructions a further time; and vi. during the further 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 a subsequent dose distribution and a subsequent profile indicative of the development of the subsequent dose distribution during its delivery, and c) compare the subsequent dose distribution to the first 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. cause the apparatus to perform the dose delivery instructions and acquire information from the detector as to a delivered radiation fluence; iv. after commencement of the sequence of instructions, perform a first computational analysis based on the acquired information as to the delivered radiation fluence in conjunction with the patient data in order to produce a first dose distribution and a first profile indicative of the development of the first dose distribution during delivery; v. subsequent to completion of the first computational analysis, cause the apparatus to perform the dose delivery instructions a further time; and vi. during the further 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 a subsequent dose distribution and a subsequent profile indicative of the development of the subsequent dose distribution during its delivery, and c) compare the subsequent dose distribution to the first 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 a subsequent dose distribution and a subsequent profile indicative of the development of the subsequent dose distribution during its delivery, and c) compare the subsequent dose distribution to the first 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.