A system for evaluating treatment parameters for a patient undergoing radiotherapy, the system comprising: (a) a register for storing: (i) a planning image corresponding to a target region of the patient, the planning image obtained prior to a radiotherapy treatment session; (ii) a portal image corresponding to the target region of the patient, the portal image obtained during the radiotherapy treatment session; and (iii) prior portal dosimetry images and corresponding assessments of the prior portal dosimetry images; and (b) a processor configured to: (i) generate a portal dosimetry image showing a difference between the planning image and the portal image; (ii) perform a similarity measurement between the portal dosimetry image and the prior portal dosimetry images; (iii) based on the similarity measurement and the assessments of the prior portal dosimetry images, determine that radiation is not being delivered as planned during the radiotherapy treatment session; and (iv) interrupt the radiotherapy treatment session when it is determined that radiation is being delivered to the wrong location, or that the target region has changed position (a) a register for storing: (i) a planning image corresponding to a target region of the patient, the planning image obtained prior to a radiotherapy treatment session; (ii) a portal image corresponding to the target region of the patient, the portal image obtained during the radiotherapy treatment session; and (iii) prior portal dosimetry images and corresponding assessments of the prior portal dosimetry images; and (i) a planning image corresponding to a target region of the patient, the planning image obtained prior to a radiotherapy treatment session; (ii) a portal image corresponding to the target region of the patient, the portal image obtained during the radiotherapy treatment session; and (iii) prior portal dosimetry images and corresponding assessments of the prior portal dosimetry images; and (b) a processor configured to: (i) generate a portal dosimetry image showing a difference between the planning image and the portal image; (ii) perform a similarity measurement between the portal dosimetry image and the prior portal dosimetry images; (iii) based on the similarity measurement and the assessments of the prior portal dosimetry images, determine that radiation is not being delivered as planned during the radiotherapy treatment session; and (iv) interrupt the radiotherapy treatment session when it is determined that radiation is being delivered to the wrong location, or that the target region has changed position (i) generate a portal dosimetry image showing a difference between the planning image and the portal image; (ii) perform a similarity measurement between the portal dosimetry image and the prior portal dosimetry images; (iii) based on the similarity measurement and the assessments of the prior portal dosimetry images, determine that radiation is not being delivered as planned during the radiotherapy treatment session; and (iv) interrupt the radiotherapy treatment session when it is determined that radiation is being delivered to the wrong location, or that the target region has changed position