1. A medical data-processing system for assisting a positioning of a first medical structure relative to a second medical structure, the system comprising a navigation system, a display, and a computer having a processor, the processor being configured to: acquire, at the processor, two-dimensional projection image data comprising two-dimensional projection image information describing a two-dimensional projection image of at least the first medical structure; acquire, at the processor, registration data comprising registration information describing a registration of the two-dimensional projection image with respect to the first medical structure; acquire, at the processor and on the basis of said two-dimensional projection image data, two-dimensional position data comprising position information describing a position of at least one base point and at least one reference point in the two-dimensional projection image; acquire, at the processor and on the basis of the two-dimensional position data, positional relationship data comprising positional relationship information describing a directional distance between the at least one base point and the at least one reference point in the two-dimensional projection image; acquire, at the processor from the navigation system, three-dimensional position data comprising position information describing a position of the at least one base point and the at least one reference point in a three-dimensional anatomical space; determine, by the processor and on the basis of the positional relationship data, the registration data and the three-dimensional position data, three-dimensional reference point correspondence data comprising correspondence information describing a directional distance between the at least one base point and the at least one reference point in the three-dimensional anatomical space that is consistent with the directional distance in the two-dimensional projection image; and output the three-dimensional reference point correspondence data to the display thereby assisting a surgeon in positioning the first medical structure relative to the second medical structure.