A fiducial marker system (210a-c) comprises a first unit and a second unit. The first unit is patient affixable and comprises a partly spherical inner surface having a first center point that is adapted to receive a touch probe measurement head for measuring the spatial position of the center point. The second unit is releasably attachable to an outer surface of the first unit, and has a second center point, wherein the first center point and the second center point are substantially identical when the second unit is attached to the first unit. For stereotactic therapy, a patient is imaged with the first and second unit assembled. Then the second unit is detached. A stereotactic frame (300) is attached to the first units via arms. Thus frameless imaging is provided in a frame based stereotactic surgery system with high precision. Repeated assembly of the frame (300) to the first units, remaining in the patient, provides advantageous fractionized stereotactic radiotherapy.