Misdiagnosis of meningeal melanomatosis with insignificant cytologic heterogeneity and its diagnostic reconsideration: a case report

Background Meningeal melanomatosis is a rare primary neoplasm of the central nervous system. According to the World Health Organization Classification of Central Nervous System Tumors, 5th Edition (WHO CNS5) criteria, its diagnosis requires diffuse growth with cerebral parenchymal invasion and/or marked cytological atypia. Cases exhibiting a “blandly invasive” phenotype—where histological features appear deceptively bland despite unequivocal parenchymal invasion—provide important clues for deepening the understanding of current diagnostic criteria. Case presentation A 38-year-old Chinese woman presented with progressively worsening headache, nausea, and vomiting. Imaging studies revealed an intracranial occupying lesion accompanied by diffuse leptomeningeal involvement. To establish a definitive diagnosis, the patient underwent intracranial lesion biopsy. The initial pathological diagnosis was meningeal melanocytosis; however, in view of the malignant biological behavior evidenced by cerebellar parenchymal involvement, the final clinical diagnosis was revised to meningeal melanomatosis. Conclusion This case demonstrates that, under the WHO CNS5 criteria, histologically confirmed parenchymal invasion alone satisfies the diagnostic requirements for meningeal melanomatosis, underscoring the critical role of an integrated clinicoradiological-pathological diagnostic approach in achieving accurate diagnosis of such rare diseases, and providing a reference conforming to current standards for refining the diagnostic framework of these tumors.