1. A method for providing lungs symmetry information, comprising the following steps: a) providing 2D X-ray image data of a patient's chest; b) segmenting the image data to provide segmented image data separated from un-segmented areas, the segmented image data comprising a lungs mask image having first and second selected regions corresponding to lung structures; c) extracting spatial lung volume information from the image data using the segmented image data derived from the image data; d) determining lungs symmetry information using the extracted spatial lung volume information; and e) providing the lungs symmetry information to a user, wherein the extracting step c) further comprises: c1) generating a thorax mask image wherein, for the first and second selected regions, the image data is replaced by applying a pixel value representing soft tissue; c2) subtracting the thorax mask image from the 2D X-ray image data to achieve a lungs-only image comprising the spatial lung volume information, and wherein the determining step d) further comprises: d1) summing the spatial lung volume information of the first and second selected regions.