A medical data processing method of determining a spatial relationship between a marker device (1, 1′, 1″, 20) and a resection plane (50, 120) associated with an anatomical structure (5, 12) of a patient's body, the marker device (1, 1′, 1″, 20) being video-detectable by an imaging unit (6), the method being constituted to be executed by a computer and comprising the following steps: a) acquiring imaging unit position data describing a predetermined spatial relationship between the imaging unit (6) and the resection plane; b) acquiring marker device position data describing a spatial relationship between the marker device (1, 1′, 1″, 20) and the imaging unit (6) based on imaging the marker device (1, 1′, 1″, 20) with the imaging unit (6) in order to generate an orientation-dependent image appearance of the marker device (1, 1′, 1″, 20); c) determining, based on the imaging unit position data acquired in step a) and the marker device position data acquired in step b) and based on the orientation-dependent image appearance of the marker device (1, 1′, 1″, 20), resection plane (50, 120) data describing the spatial relationship between the resection plane (50, 120) and the marker device (1, 1′, 1″, 20).