MR Imaging Findings of the Intraspinal Meningeal Melanocytoma: Correlation with Histopathologic Findings

Discussion

Melanocytes originate from the neural crest and are normally found in the leptomeninges.^2,4 These cells are present in the leptomeninges of healthy adults and may result in primary leptomeningeal pigmented tumors, including pigmented meningioma, malignant melanoma, MM, melanotic schwannoma, and melanoblastosis.^6,7 Malignant melanoma and MM are of the same origin and represent the malignant and benign ends of the tumor spectrum, respectively.⁵

MM was first coined by Limas and Tio³ in 1972. It is a very rare, pigmented, and slow-growing benign neoplasm.⁸ However, malignant transformation of melanocytoma to malignant melanoma has been reported.^5,9 To date, approximately 112 cases of MM have been reported,⁵ and most were found in the cerebrum. The tumor occurs almost exclusively in the posterior fossa and spinal cord.^6,7,10,11 The annual incidence of MM is estimated to be 1 case per 10 million people, and women are vulnerable.¹⁰ Our hospital has an extremely large collection area for patients from Guangdong province and 4 neighboring provinces. The orthopedics center in our hospital is very famous and is also a referral center for spinal tumors. Therefore, we have collected 5 cases of patients with MM in the past 7 years. The incidence is not affected by race or genetic risk factor, and there are no literature reports about MM, to our knowledge. MM may occur at any age but is most commonly seen in the fifth decade. The average age in our series is 49.2 years, which is consistent with that in other reports.^10,11

Approximate 59 cases of intraspinal MM have been reported.⁵ Nineteen cases were founded in the intramedullary location, while others were located in the extramedullary area.^5,9 The clinical signs and symptoms depend on its location and may present as myelopathy and seizures.^10,12 The symptoms of our patients were nonspecific, mainly caused by mass compression.

Most MMs are commonly found as a solitary lesion, but they can also present as multifocal or disseminating along the arachnoid and dura mater.^13,14 To date, approximately 4 cases with multifocal lesions have been reported.^15,16 Ali et al⁶ considered multifocal MM as a distinct pathologic entity that seems to portend an aggressive clinical course and a poor prognosis, even when malignant pathologic features are absent.

In our study, 2 patients with multifocal lesions did not show malignant pathologic features compared with those with solitary lesions, but 1 patient with multifocal lesions developed malignant melanoma and died of metastasis 13 months after surgery. It has been reported that MM might transform into malignant melanoma^9,17 and then cause distant metastasis.²⁸ Our study also supports multifocal MM possibly predicting a poor outcome.

MR imaging is the first choice for spinal tumors. On MR imaging, MM is generally an attenuated extra-axial lesion attached to the dura, which may not have a distinct margin.⁵ It is typically hyperintense on T1WI and hypointense on T2WI and enhances homogeneously with gadolinium.^{10,16⇓⇓–19} The characteristic signal intensity of MM is thought to be caused by the paramagnetic susceptibility effect of the melanin pigment or hemorrhagic foci; hence, its radiologic signal intensity is variable and inconstant.^16,19,20 However, differences in duration, different amounts of melanin, and hemorrhage in malignant melanoma may influence the signal intensity of tumor cells on MR imaging.^9,14,19,20

Amelanotic melanoma and melanoma without hemorrhage are iso- or hypointense on T1WI and moderately hyperintense on T2WI.^21⇓–23 Isiklar et al²⁴ reviewed the MR images of 30 patients with intracerebral melanoma and sorted these into 4 groups: melanotic pattern (hyperintense on T1WI, hypointense on T2WI), amelanotic pattern (hypointense or isointense on T1WI and hyperintense or isontense on T2WI), indeterminate or mixed pattern (MR imaging characteristics not conforming to the first 2 categories), and hematoma pattern (MR imaging exhibiting only hematoma characteristics).²⁷ In our study, cases with >10% of melanin-containing cells showed a melanotic pattern, while case 2 did not have typical MR imaging and was then proved to have <10% melanin-containing cells by histopathology.

Wadasadawala et al²² considered the following aspects useful for the differential diagnosis of MM: 1) Location: Pigmented meningioma is located in the extra-axial area; melanotic schwannoma is located in the cerebellopontine angle; MM and primary melanoma are often located in the high cervical cord or perimedullary area, whereas metastatic melanoma is located in the supratentorial or gray-white matter junction. 2) Number: The lesion of pigmented meningioma, MM, or primary melanoma is often single, while metastatic melanoma is multiple. 3) Hemorrhage: The pigmented meningioma and melanotic schwannoma do not have a hemorrhagic focus, while MM and primary and metastatic melanoma often have it.²² 4) Dural involvement: Dural involvement and the “dural tail” sign are often observed in meningioma, MM, or meningeal melanocytosis, but metastatic melanoma seldom presents dural involvement.

Because the differential diagnosis of MM from other pigmented malignancies is difficult on the basis of only the imaging information, a final diagnosis is dependent on the results of pathologic examination. Grossly, MM usually presents as a solitary well-circumscribed black mass, while lesions in red, white, tan, brown, or blue have been reported.^10,23,24 MMs may be mistaken for meningioma at gross inspection because they are attached to the meninges. MM cells tend to compress adjacent tissues instead of infiltrating them. However, local invasion of neural structures has been reported in some cases.^6,25,26 The cytologic feature of MM is uniform, lacking in atypia and mitotic activity and showing different degrees of melanization.^{12,27⇓⇓–30}

IHC staining is always indispensable for differentiating MM from other pigmented tumors. MM is characterized by a positive reaction to HMB-45, vimentin, and S-100 proteins¹² and a negative reaction to EMA and Leu-7.^7,20 Meningioma cells lack the expressions of HMB-45 and S-100, and a positive reaction to EMA differentiates pigmented meningioma from MM.^7,12 The absence of a reaction to Leu-7 excludes the diagnosis of melanocytic schwannoma.^7,28 Vimentin usually appears in MM but is only rarely present in malignant melanoma. Ultrastructurally, desmosomes are characteristic of meningioma but are generally absent in MM.^3,5,19 However, a differential diagnosis between MM and malignant leptomeningeal melanoma may present some difficulties.²⁰ Because the histologic characteristics of MM and melanoma are overlapping, thorough physical and radiologic examinations of the patient are required to exclude a primary cutaneous, ocular, or mucosal melanoma.^5,10