The present invention relates to an apparatus for determining a modification of a size of an object. The apparatus comprises a registration unit (13) for registering with respect to each other a first region of interest in a first image data set showing an object at a first time and a second region of interest in a second image data set showing the object at a second time being different from the first time, wherein the registration unit is adapted to generate a scaling value by performing at least a scaling transformation for registering the first region of interest and the second region of interest with respect to each other. The apparatus further comprises a modification value determination unit (14) for determining a modification value, which indicates the modification of the size of the object, depending on the generated scaling value.
