A computer-implemented data processing method for providing variation data which describe a non-enhancing tumour, 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 the non-enhancing tumor or which includes the non-enhancing tumor in a different physical state than in the second anatomical part; providing (S30, S40) 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 providing the variation data on the basis of the position change data; a) wherein the description of the positional changes provided by the position change data comprises change positions (r) which define the positions at which the positional changes occur, and the method comprises the steps of: determining the positions at which the change in position is at a maximum; determining the variation data by identifying a delineation of the non-enhancing tumor on the basis of said positions at which the change is at a maximum, wherein the variation data comprise a description of a position and geometry of the delineation of the non-enhancing tumor such that the positional changes are at said maximum at the delineation; or b) wherein: the positional changes are represented by a vector field (F(r)); and the step of determining the variation data involves calculating a determinant of a Jacobian matrix of the vector field (F(r)) and of determining a position and geometry of a surface of the non-enhancing tumor such that said surface is defined by the positions at which the determinant of the Jacobian matrix is equal to 1. 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 the non-enhancing tumor or which includes the non-enhancing tumor in a different physical state than in the second anatomical part; providing (S30, S40) 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 providing the variation data on the basis of the position change data; a) wherein the description of the positional changes provided by the position change data comprises change positions (r) which define the positions at which the positional changes occur, and the method comprises the steps of: determining the positions at which the change in position is at a maximum; determining the variation data by identifying a delineation of the non-enhancing tumor on the basis of said positions at which the change is at a maximum, wherein the variation data comprise a description of a position and geometry of the delineation of the non-enhancing tumor such that the positional changes are at said maximum at the delineation; or b) wherein: the positional changes are represented by a vector field (F(r)); and a) wherein the description of the positional changes provided by the position change data comprises change positions (r) which define the positions at which the positional changes occur, and the method comprises the steps of: determining the positions at which the change in position is at a maximum; determining the variation data by identifying a delineation of the non-enhancing tumor on the basis of said positions at which the change is at a maximum, wherein the variation data comprise a description of a position and geometry of the delineation of the non-enhancing tumor such that the positional changes are at said maximum at the delineation; or determining the positions at which the change in position is at a maximum; determining the variation data by identifying a delineation of the non-enhancing tumor on the basis of said positions at which the change is at a maximum, wherein the variation data comprise a description of a position and geometry of the delineation of the non-enhancing tumor such that the positional changes are at said maximum at the delineation; or b) wherein: the positional changes are represented by a vector field (F(r)); and the step of determining the variation data involves calculating a determinant of a Jacobian matrix of the vector field (F(r)) and of determining a position and geometry of a surface of the non-enhancing tumor such that said surface is defined by the positions at which the determinant of the Jacobian matrix is equal to 1.