Case of the Week
Section Editors: Matylda Machnowska1 and Anvita Pauranik2
1University of Toronto, Toronto, Ontario, Canada
2BC Children's Hospital, University of British Columbia, Vancouver, British Columbia, Canada
Sign up to receive an email alert when a new Case of the Week is posted.
July 18, 2024
Malignant Melanotic Nerve Sheath Tumor (MMNST)
- Background:
- Malignant melanotic nerve sheath tumor (MMNST), which was formerly known as "melanotic schwannoma," is an uncommon type of nerve sheath tumor, comprising less than 1% of all such tumors. It predominantly affects the posterior spinal nerve roots and paraspinal ganglia. The tumor consists of Schwann cells that can produce melanin in varying amounts and have the potential to metastasize.
- MMNST was previously known as a rare variant of schwannoma but has since been recognized as a distinct entity with its unique molecular and methylation clustering. The tumors are officially codified as MMNST according to the updated 5th edition (2021) of the WHO Classification of CNS Tumors. It is commonly associated with PRKAR1A mutations and Carney complex, a genetic disorder characterized by multiple neoplasia.
- Clinical Presentation:
- The clinical manifestation of MMNSTs usually depends on the tumor's location. For instance, in a cervical spine location, patients might exhibit symptoms of cord compression or specific symptoms affecting both sensory and motor functions. The tumor's behavior is inconsistent, with a propensity for recurrence and metastasis. Larger peripheral lesions might simply appear as palpable lumps.
- Key Diagnostic Features:
- Intrinsic hyperintense signal is present on T1-weighted MR images and hypointense signals on T2-weighted images.
- The tumors also typically enhance with contrast.
- Histologically, tumors are characterized by spindle-shaped cells with melanin pigmentation.
- Differential Diagnosis:
- Intradural differential diagnoses:
- Schwannoma: This benign nerve sheath tumor can appear identical, especially if the melanin content in the MMNSTs is small.
- Meningioma: A frequently occurring benign tumor that originates from the meninges. However, these are usually dural-based and lack the melanin pigmentation.
- Myxopapillary ependymoma: Although very rare, this variant of ependymoma can present with similar features. However, these are typically intramedullary tumors and can be distinguished by their location within the spinal cord.
- Intramedullary tumors (such as astrocytoma or ependymoma): These tumors are usually located within the spinal cord itself and do not contain melanin. They may present with similar symptoms due to mass effect.
- Extradural differential diagnoses:
- Metastasis (particularly melanoma): Metastatic spinal cord malignancy occurs in a small percentage of patients with cancer. These can be distinguished by the MRI appearance of enhanced nodular foci with surrounding edema, pial enhancement, and fludeoxyglucose uptake on PET-CT.
- Giant cell tumor: These are generally benign, but locally aggressive tumors. They are rare in the spine but should be considered in the differential diagnosis.
- Intradural differential diagnoses:
- Treatment:
- The primary approach to MMNST involves surgical removal. If total removal is unattainable due to the tumor's location or patient-specific conditions, postoperative radiation therapy might be an option. Regular monitoring is crucial because of the potential for the tumor to become malignant and recur.
Current Issue
Case Collections
