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Meningeal melanocytomas are rare benign primary melanocytic tumors of the CNS that are derived from leptomeningeal melanocytes. They can occur anywhere along the neuraxis but are most commonly found in the spinal canal near the foramen magnum, as well as the posterior cranial fossa, Meckel cave, or adjacent to cranial nerve nuclei 3,4.
Peak presentation is in the fourth and fifth decades, although these tumors have been diagnosed in all age groups 1,4. Occurrence in children is very rare 1.
Clinical presentation relates to tumor location and size. Spinal meningeal melanocytomas (most common) typically present with progressive pain, weakness, and sensory deficits. Patients rarely present with subarachnoid hemorrhage.
When these tumors are found in the trigeminal cave, then they are associated with a nevus of Ota, a benign dermal melanocytic nevus involving the ophthalmic (Va) and maxillary (Vb) divisions of the trigeminal nerve (CN V) 4,5.
Meningeal melanocytomas are most commonly found in the cervical and thoracic regions (intrathecal-extramedullary). Within the spine, melanocytomas present as intradural masses, and maybe intradural extramedullary or rarely intramedullary. They are most commonly found in the upper cervical region, as melanocytes are most concentrated at this location 3. Less commonly they occur in the intracranial compartment 4.
Macroscopically, meningeal melanocytomas are circumscribed tumors. The degree of pigmentation is variable, ranging from black to non-pigmented 4.
On light microscopy, there are spindle, fusiform, epithelioid, or polygonal cells without evidence of anaplasia. The cells have eosinophilic cytoplasm with a variable content of melanin pigment. Mitotic figures are rare or absent and necrosis and hemorrhage are generally not seen 1,4. These features help in differentiating melanocytomas from metastatic or primary melanomas of the central nervous system.
- HMB45: positive
- melan-A: positive
- MITF: positive
- S100: positive
- vimentin: variable
- neuron-specific enolase: variable
- GFAP, NFPs, cytokeratins, EMA: usually negative
The preoperative diagnosis of meningeal melanocytoma is often difficult, as the clinical and neuroradiological features of the tumor are non-specific 2.
Well-defined, isodense to hyperdense, homogeneous, contrast-enhancing lesion 1.
The MRI appearance of meningeal melanocytomas is variable, depending on the amount of melanin content present.
Signal characteristics include:
- T1: isointense or hyperintense
- T2: isointense or hypointense
- T1 C+ (Gd): heterogeneous enhancement
- T2* GRE: may show blooming of low signal
Treatment and prognosis
Although classified as benign, meningeal melanocytomas may behave aggressively and a limited number may transform to malignant melanomas 1-4. Complete excision is the treatment of choice, however, this often not possible as intra-operative hemorrhage may be severe. Furthermore, local recurrence has been reported even after gross total removal 1. Due to the risk of tumor recurrence even after complete excision, adjuvant radiation therapy is advised in cases of both complete and incomplete resection 2.
Imaging differential considerations include:
- other melanotic lesions
- hemorrhagic lesions
- hemorrhagic metastases
- rounded regions of heterogeneous signal intensity on T1 and T2 weighted images due to blood products of varying ages (“popcorn” appearance)
- hemosiderin rim on T2 weighted images
- hypointense “blooming” on gradient echo sequences
- minimal to no enhancement
In the spine, in addition to the lesions above, consider: