Primary intramedullary melanocytoma - thoracic spine


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 tumors of the CNS can manifest as solid masses or as diffuse dissemination within the subarachnoid space. They range in histologic grade from benign to malignant, differentiating between the following entities 1:

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 melanization 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.