Clinical practice is increasingly replacing traditional open surgical procedures with minimally invasive techniques for cancer diagnosis and therapy. These procedures typically require placement of a surgical instrument into the organ of interest with a high degree of accuracy. In general, the success of a treatment or of a diagnosis is highly dependent on the accuracy of instrument insertion and thus depends crucially on the skills and the experience of the physician. While intra-operative navigation has been proven to be highly effective in interventions on bony or sufficiently rigid...