Primary Melanocytic Neoplasms



Primary Melanocytic Neoplasms


Elisabeth J. Rushing, MD





Key Facts


Terminology



  • Spectrum from low-grade melanocytoma to malignant melanoma



    • No consensus grading system


Clinical Issues



  • Sites



    • Posterior fossa, spinal cord, supratentorial


  • Prognosis



    • Good for melanocytoma


    • Guarded for intermediate lesions


    • Poor for melanoma, but better than for metastatic melanoma to CNS


Image Findings



  • Heavily pigmented cases are dark on T2WI and bright on pre-contrast T1WI


Microscopic Pathology



  • Mitotic activity minimal or absent in melanocytoma


  • Nested architecture, in some cases, especially lower grade


  • Meningioma-like, sheet-like, or lobular architecture in some cases


  • Spindled or epithelioid cells more likely in melanoma


  • Nucleoli small in most melanocytomas; large, eosinophilic in melanomas, but also in some melanocytomas and intermediate lesions


  • Necrosis and invasion in melanoma


  • Ill-defined intermediate category


Top Differential Diagnoses



  • Melanotic schwannoma, meningioma







On T1WI (left), primary intramedullary melanocytoma of the spinal cord shows intrinsic T1 shortening characteristic of melanin image. T2WI (right) shows classic hypointensity image. (Courtesy P. Hildenbrand, MD.)






Tight nests of bland, slightly spindled cells are key features of melanocytoma. Sparsely pigmented or amelanotic tumors may be misinterpreted as meningioma.


TERMINOLOGY


Definitions



  • Uncommon neoplasms derived from normal leptomeningeal melanocytes



    • Nodular mass



      • Morphologic spectrum from low-grade melanocytoma to melanocytic tumor of intermediate differentiation to malignant melanoma


      • No consensus on precise criteria distinguishing these subdivisions


    • Diffuse leptomeningeal infiltrate



      • Meningeal melanocytosis/melanomatosis (neurocutaneous melanosis) (rare)


ETIOLOGY/PATHOGENESIS


Developmental Anomaly



  • Neurocutaneous melanosis



    • Aberrant expression of hepatocyte growth factor/scatter factor (HGF) implicated


CLINICAL ISSUES


Epidemiology



  • Incidence



    • Rare


  • Age



    • Mostly adults



      • 5th decade for focal malignant melanoma


      • 4th decade for diffuse melanomatosis


    • Children with neurocutaneous melanosis syndrome



      • Symptomatic by 2 years of age


  • Gender



    • Primary CNS nodular melanoma more common in males


    • No preference for neurocutaneous melanosis


  • Ethnicity



    • Caucasians > other races


Site



  • Posterior fossa



    • Base of skull


    • Cerebellopontine angle


    • Meckel cave



      • Melanocytoma in this site associated with nevus of Ota


  • Spinal cord



    • Intradural extramedullary



      • Most melanocytomas arise at cervical and thoracic levels


    • Largely intramedullary (rare)


  • Supratentorial



    • Leptomeninges


    • Intraventricular (rare)


Presentation



  • Nodular mass



    • Site-dependent deficits


  • Diffuse leptomeningeal infiltrate



    • Seizures


    • Signs/symptoms of increased intracranial pressure


    • Myelopathy with spinal cord involvement


Treatment



  • Complete surgical resection for melanocytoma


  • No effective therapy for others


Prognosis



  • Good for melanocytoma



    • Rarely undergo malignant transformation


  • Guarded for melanocytic tumors of intermediate differentiation


  • Poor for melanoma, but



    • Better prognosis for primary CNS than metastatic melanoma


  • Poor for neurocutaneous melanosis




    • Majority undergo malignant transformation with death by 4 years of age


IMAGE FINDINGS


MR Findings



  • Nodular mass



    • Extraaxial and well circumscribed


    • Iso-, hypointense on T2WI and hyperintense on pre-contrast T1WI


    • Homogeneous enhancement


    • T1 and T2 signal characteristics dependent on melanin content



      • Highly pigmented lesions bright on pre-contrast T1WI and dark on T2WI


      • Most lesions not sufficiently pigmented to generate these signal characteristics • Diffuse leptomeningeal infiltrate


    • T1 shortening within brain parenchyma & meninges


    • Leptomeningeal or intraparenchymal enhancement heralds malignant transformation


MACROSCOPIC FEATURES


General Features



  • Nodular mass



    • Solitary mass lesions, pigmented or nonpigmented


    • Hemorrhage or necrosis in some cases


  • Diffuse leptomeningeal infiltrate



    • Dense, black subarachnoid staining

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Jul 8, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Primary Melanocytic Neoplasms

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