An apparatus for evaluating a change in radiation distribution within a target tissue of a patient, the apparatus constituted of: a memory having stored thereon parameters of radiation to be output by an irradiation source, the radiation parameters determined responsive to a reference image of the patient target tissue a processor in communication with the memory and an output module in communication with the processor, wherein the processor is arranged to: receive a present image of the patient target tissue determine the expected distribution of radiation within the patient target tissue for an irradiation incidence according to the stored radiation parameters determine the difference between the determined distribution and a predetermined distribution parameter and control the output module to output a signal responsive to the determined difference.
