A method and apparatus for lesion or organ definition for the purpose of radiation treatment planning localization and treatment position verification. The apparatus uses a combination of an ultrasound imaging system and a diagnostic imaging system to acquire localization ultrasound images referenced in the coordinate space of the diagnostic imaging system through the use of a position sensing system. The method compares the location of the lesion in the localization ultrasound images with the position of the lesion in ultrasound images taken while the patient lies on the treatment table of a therapy treatment unit, suggests corrective measures to place the lesion in its intended treatment position and executes the correction upon confirmation from qualified personnel.