A case of late onset multiple sclerosis mimicking glioblastoma and displaying intraoperative 5-aminolevulinic acid fluorescence NORBERT MANZO1, ULF NESTLER1, CARAÏBES.NEUROSURGERY, Martinique Various case reports and case series describe the resemblance of multiple sclerosis lesions to cerebral glioma on MRI. Multiple sclerosis accounts for about 1% of histological diagnosis in biopsies or surgical specimens resected for suspicion of brain tumor. Only few information is available about the interaction between sclerotic plaques and the fluorescence guided resection agent 5-aminolevulinic acid, which is currently used more and more often to allow for complete resection of malignant brain tumors. We report the case of a 57-year-old male patient who underwent neurosurgical intervention for an intracerebral mass lesion with the features of a malignant brain tumor on preoperative MRI. During intervention, the lesion showed strong 5-Ala fluorescence and intraoperative histological diagnosis on frozen material was malignant astrocytoma. After additional examinations on the paraffin embedded specimens definitive diagnosis revealed multiple sclerosis. Conclusion: We hypothesize that the inflammatory lymphatic cells engaged in the demyelinating process are susceptible to protoporphyrin IX accumulation induced by 5-aminolevulinic acid. In this way a fluorescence signal, very similar to the signal of malignant tumor cells, becomes detectable.