An intervention system employs an optical shape sensing tool (32) (e.g., a brachytherapy needle having embedded optical fiber(s)) and a grid (50, 90) for guiding an insertion of the optical shape sensing tool (32) into an anatomical region relative to a grid coordinate system. The intervention system further employs a registration controller (74) for reconstructing a segment or an entirety of a shape of the optical shape sensing tool (32) relative to a needle coordinate system, and for registering the needle coordinate system to the grid coordinate system as a function of a reconstructed segment/entire shape of the optical shape sensing tool (32) relative to the grid (50, 90) (i.e., reconstruction of a segment/entire shape of the OSS needle inserted into/through the grid serving as a basis for the grid/needle coordinate system registration).