A system for evaluating treatment parameters for a patient undergoing radiotherapy is provided. The system determines a portal dosimetry image (54, 56) showing a difference between a planning image and a portal image (50) both images corresponding to a target region of the patient. The planning image is obtained prior to a radiotherapy treatment session. The portal image (50) is obtained during the radiotherapy treatment session. The system performs a similarity measurement between the portal dosimetry image and prior portal dosimetry images stored in the system. Based on the similarity measurement and the assessments of the prior portal dosimetry images, the system is able to determine when radiation is not being delivered as planned during the radiotherapy treatment session. The system interrupts the radiotherapy treatment session if such determination is made.
