Safety and accuracy of stereotactic brain tumor biopsies can be improved by integrating a fiber-optical probe into the biopsy needle. After administering 5-aminolevulinic acid, the fluorophore protoporphyrin IX (PpIX) is accumulated in vital tumor tissue, which may then be localized via fluorescence detection. Apparently, blood covered tumor tissue only produces a non-vanishing signal when excited with red light around 633 nm. By illuminating the tissue that is to be sampled and comparing the remitted intensities at two wavelengths with strongly differing hemoglobin absorption, blood vessels can be recognized up to 800-1200 µm ahead of the probe to avoid intracerebral hemorrhage.