An apparatus for determining a modification of a size of an object (23, 24), the apparatus comprising: - an image data set providing unit for providing a first image data set showing the object at a first time and for providing a second image data set showing the object at a second time being different from the first time, - a region of interest providing unit for providing a first region of interest, in which the object shown in the first image data set is located, in the first image data set and for providing a second region of interest, in which the object shown in the second image data set is located, in the second image data set, - a registration unit for registering the first region of interest and the second region of interest with respect to each other, 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, wherein the registration unit is configured for generating a first scaling value by performing the scaling transformation for registering the first region of interest onto the second region of interest and generating a second scaling value by performing the scaling transformation for registering the second region of interest onto the first region of interest, - a modification value determination unit for determining a modification value, which indicates the modification of the size of the object (23, 24), depending on the generated scaling value, wherein the modification value determination unit is configured for determining a first modification value, which indicates the modification of the size of the object, depending on the generated first scaling value and determining a second modification value, which indicates the modification of the size of the object, depending on the generated second scaling value, - a similarity determination unit for determining whether one of the first modification value and the second modification value and the inverse of the other of the first modification value and the second modification value are similar with respect to a similarity measure, and - an output unit for outputting a signal if the similarity determination unit has determined that one of the first modification value and the second modification value and the inverse of the other of the first modification value and the second modification value are not similar with respect to the similarity measure. - an image data set providing unit for providing a first image data set showing the object at a first time and for providing a second image data set showing the object at a second time being different from the first time, - a region of interest providing unit for providing a first region of interest, in which the object shown in the first image data set is located, in the first image data set and for providing a second region of interest, in which the object shown in the second image data set is located, in the second image data set, - a registration unit for registering the first region of interest and the second region of interest with respect to each other, 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, wherein the registration unit is configured for generating a first scaling value by performing the scaling transformation for registering the first region of interest onto the second region of interest and generating a second scaling value by performing the scaling transformation for registering the second region of interest onto the first region of interest, - a modification value determination unit for determining a modification value, which indicates the modification of the size of the object (23, 24), depending on the generated scaling value, wherein the modification value determination unit is configured for determining a first modification value, which indicates the modification of the size of the object, depending on the generated first scaling value and determining a second modification value, which indicates the modification of the size of the object, depending on the generated second scaling value, - a similarity determination unit for determining whether one of the first modification value and the second modification value and the inverse of the other of the first modification value and the second modification value are similar with respect to a similarity measure, and - an output unit for outputting a signal if the similarity determination unit has determined that one of the first modification value and the second modification value and the inverse of the other of the first modification value and the second modification value are not similar with respect to the similarity measure.