A reference image is used to calibrate a subsequent image, such as one obtained during therapy. A number of local histograms are prepared for each image, a transformation is identified that transforms the histograms to be substantially the same and is applied to voxels in the subsequent image. Changes between the reference and subsequent images such as the movement of internal structures will be preserved since the method compares local histograms and this corrects intensities, not shapes. Gently varying artifacts such as the cup artifact will be substantially eliminated. The volume image thus derived can then be used for treatment planning.