Tumefactive demyelinating lesion diagnosed by intraoperative fluorescence-guided endoscopic biopsy using 5-ALA: An illustrative case report and literature review

• Use of 5-ALA-guided endoscopy to diagnose tumefactive demyelinated lesion. • 5-ALA–guided biopsy enabled precise sampling of a deep frontal lobe lesion. • Fluorescence was present despite the non-neoplastic, demyelinating pathology. • Technique offers a less invasive, accurate diagnostic option for TDL. Tumefactive demyelinating lesions (TDLs) are inflammatory demyelinating lesions larger than 2 cm that can mimic malignant brain tumors on imaging, often necessitating histopathological confirmation. While 5-aminolevulinic acid (5-ALA) fluorescence has been reported in non-neoplastic lesions, including demyelinating diseases, its application to guide neuroendoscopic biopsy for TDL has not been previously documented. A 42-year-old man presented with progressive right hemiparesis. MRI revealed a 3-cm gadolinium-enhancing subcortical lesion in the left frontal lobe. Serological and cerebrospinal fluid workup was unremarkable, and a neoplastic lesion could not be fully excluded. A 5-ALA-guided neuroendoscopic biopsy was performed via a frontal burr hole using preoperative tractography to plan a function-preserving trajectory. Strong porphyrin fluorescence enabled targeted tissue sampling. Histopathology demonstrated findings consistent with inflammatory demyelination, and the patient was diagnosed with idiopathic TDL. The patient improved markedly following steroid pulse therapy and plasma exchange. All previously reported 5-ALA fluorescence cases in TDLs employed craniotomy or stereotactic biopsy; to our knowledge, this is the first case utilizing neuroendoscopic biopsy. Neuroendoscopy offers lower invasiveness than craniotomy while retaining the advantage of direct fluorescence-guided visualization, enabling targeted sampling of the most metabolically active areas within the lesion. Even when lesions involve eloquent regions such as the premotor cortex, preoperative tractography allows safe trajectory planning. 5-ALA fluorescence may thus improve diagnostic yield during minimally invasive neuroendoscopic procedures for TDLs, though it cannot differentiate demyelinating from neoplastic lesions, and further studies remain warranted.