1. A data processing method for providing variation data which describe a non-enhancing tumor, comprising the steps of: providing second image data which describe a second image of a region of an anatomical body, wherein the region includes a second anatomical part which includes the non-enhancing tumor; providing first image data which represent a first image of the same region, wherein said same region includes a first anatomical part which does not include non-enhancing tumor or which includes non-enhancing tumor in a different physical state than in the second anatomical part; providing position change data which describe positional changes of corresponding image elements between the first image and the second image, on the basis of the first and second image data; and a) wherein the description of the positional changes provided by the position change data comprises change positions which define the positions at which the positional changes occur, and the method comprises the steps of: determining the position at which the change in position is at a maximum; determining the variation data, in particular a delineation of the non-enhancing tumor, on the basis of the positions at which the change is at a maximum, wherein the variation data comprise a description of a position and geometry of a delineation of the non-enhancing tumor; or b) wherein: the positional changes are represented by a vector field; and the step of determining the variation data involves calculating a determinant of a Jacobian matrix of the vector field and of determining a position and geometry of a surface of the non-enhancing tumor by setting the determinant equal to 1.