Primary intramedullary melanocytoma of the thoracic spine
Patient presented with a 2-year history of progressive paraparesis, paresthesia, and dysesthesia in the left lower limb. Neurological examination demonstrated generalized hyperreflexia and clonus of the left foot.
Spinal 1.5 Tesla MRI
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Solitary well-defined fusiform intramedullary lesion involving the spinal cord at the D8-D11 level, hypointense on T2WI, moderate hyperintense on T1WI, and presenting homogeneous enhancement of its solid component after gadolinium administration.
Lesion associates a cystic component in its cranial pole, and presents susceptibility artifacts in the T2WI-GRE corresponding to its melanin component and the presence of degraded blood products.
T2WI hyperintense per focal tumoral edema at the D5-D7 levels and conus medullaris is noted, suggestive of myelopathy.
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Radiological differential diagnosis includes:
- primary or metastatic malignant melanoma
- melanotic schwannoma
- melanotic meningioma
- cavernous malformation
Tumor resection was performed through osteoplastic laminectomy and under electrophysiologic intraoperative monitoring. Tumor fragments were soft, brownish and hemorrhagic.
Spinal cord excision
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H&E stained slides show sheets and nests of cells with marked melanin pigment deposition. Immunohistochemistry study revealed positivity for melanocytic markers Melan A and HMB45. MIB-1 (Ki-67) proliferative index of 5%.
Notes: The case is submitted in collaboration with Dr. Jose Luís León Guijarro and Dr. Liria Terrádez Mas.
Melanocytes are normal, neural crest-derived cells present in the human leptomeninges (pia and arachnoid membranes) primarily at the base of the brain, the posterior fossa, and around the upper
cervical spinal cord.
Primary melanocytic tumours of the CNS can manifest as solid masses or as diffuse disseminations within the subarachnoid space. They range in histologic grade from benign to malignant, differentiating between the following entities 1:
- meningeal melanocytoma: benign mass-forming proliferation
- meningeal melanoma: malignant mass-forming proliferation
- meningeal melanocytosis: diffuse meningeal benign melanocytic proliferation
- meningeal melanomatosis: diffuse meningeal malignant melanocytic proliferation
Melanocytomas are more common in women (mean age 45-50 years old). A prolonged evolution of clinical signs of myelopathy or radiculopathy prior to surgical resection (from 5 to 10 years) has been documented 2,3.
Approximately 100 cases of melanocytomas have been reported in the CNS (brain and spinal cord) since Limas and Tio coined the term "meningeal melanocytoma" in 1972. In the spinal cord, most cases of melanocytomas are found in the extramedullary intradural compartment, at the cervical and thoracic spinal levels. An intramedullary location as our case depicts is extremely rare, with only 24 cases reported before 2,6.
The unique paramagnetic properties of melanin results in a relatively specific MRI pattern for melanocytoma, consisting of iso- or hyperintensity on T1WI and iso- or hypointensity on T2WI, and with homogeneous enhancement. The differences in MRI signal intensities relate to a variable degree of tumor melanisation 1-6.
Definitive diagnosis is based solely on histopathological and immunohistochemical examination.
The distinction between melanocytoma and melanoma rests on the identification of cytologic atypia, mitotic activity, necrosis, and neural parenchymal invasion.
MIB-1 (Ki-67) labeling index seen in melanocytomas is low (0%-2%) while in primary melanomas is higher (2%-15%). Based on a proposition by Brat et al.1,7 the WHO classification assigns an intermediate grade to melanocytomas with increased mitotic activity and infiltrative growth that fail to meet all characteristics of malignant melanoma. In our case, MIB-1 (Ki-67) proliferation index was 5% and definitive diagnosis of intramedullary Intermediate-grade melanocytoma of the thoracic spine was made.
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