A system for evaluating treatment parameters for a patient undergoing radiotherapy, the system comprising: a register configured to store a planning image corresponding to a target region of the patient, the planning image obtained prior to a radiotherapy treatment session, a portal image corresponding to the target region of the patient, the portal image obtained during the radiotherapy treatment session, and prior portal dosimetry images and corresponding assessments of the prior portal dosimetry images; and a processor configured to: generate a portal dosimetry image (54, 56, 60, 70, 80) showing a difference between the planning image and the portal image, perform a similarity measurement between the portal dosimetry image (54, 56, 60, 70, 80) and the prior portal dosimetry images, and based on the similarity measurement and the assessments of the prior portal dosimetry images, provide an indication that the difference between the planning image and the portal image is either a true positive difference or a false positive difference, the true positive difference indicating that radiation was not delivered as planned during the radiotherapy treatment session, and the false positive difference indicating that radiation was delivered as planned during the radiotherapy treatment session. a register configured to store a planning image corresponding to a target region of the patient, the planning image obtained prior to a radiotherapy treatment session, a portal image corresponding to the target region of the patient, the portal image obtained during the radiotherapy treatment session, and prior portal dosimetry images and corresponding assessments of the prior portal dosimetry images; and a planning image corresponding to a target region of the patient, the planning image obtained prior to a radiotherapy treatment session, a portal image corresponding to the target region of the patient, the portal image obtained during the radiotherapy treatment session, and prior portal dosimetry images and corresponding assessments of the prior portal dosimetry images; and a processor configured to: generate a portal dosimetry image (54, 56, 60, 70, 80) showing a difference between the planning image and the portal image, perform a similarity measurement between the portal dosimetry image (54, 56, 60, 70, 80) and the prior portal dosimetry images, and based on the similarity measurement and the assessments of the prior portal dosimetry images, provide an indication that the difference between the planning image and the portal image is either a true positive difference or a false positive difference, the true positive difference indicating that radiation was not delivered as planned during the radiotherapy treatment session, and the false positive difference indicating that radiation was delivered as planned during the radiotherapy treatment session. generate a portal dosimetry image (54, 56, 60, 70, 80) showing a difference between the planning image and the portal image, perform a similarity measurement between the portal dosimetry image (54, 56, 60, 70, 80) and the prior portal dosimetry images, and based on the similarity measurement and the assessments of the prior portal dosimetry images, provide an indication that the difference between the planning image and the portal image is either a true positive difference or a false positive difference, the true positive difference indicating that radiation was not delivered as planned during the radiotherapy treatment session, and the false positive difference indicating that radiation was delivered as planned during the radiotherapy treatment session.