A body part is scanned to produce a first set of imaging data. A target lesion in the imaging data is identified. The body part is rescanned at a subsequent time so as to produce a second set of imaging data. The target lesion is identified in the second set of imaging data and the size of the target lesion is measured in the first and second sets of imaging data to determine two apparent image volumes corresponding to the first and second sets of imaging data. A change in size is estimated by comparing the first and second apparent lesion sizes. A variance on the change in size is estimated so as to determine a bound on the change in size measurement.