FINAL DIAGNOSIS: METASTASIZING, MELANOCYTIC LEPTOMENINGEAL TUMOR WITH HISTOMORPHOLOGICAL CHARACTERISTICS OF A MELANOCYTOMA.
We report a case in which the tumor morphologically resembles a benign melanocytoma, but which showed local recurrence as well as cerebral and hepatic metastases. To the best of our knowledge, this is the first case of a melanocytoma associated with hepatic metastasis.
Meningeal melanocytomas occur at every age, are usually unilocular and appear as nodes attached to the dura mater. They are mainly located in the posterior cranial fossa or the spinal canal [4,11]. Imaging studies of these tumors show homogeneous contrast enhancement .
Histologically, melanocytomas are hypercellular with cells arranged in nests or sheets. They have oval nuclei, prominent nucleoli and finely dispersed chromatin. Mitoses are rare. Necrosis, hemorrhage and CNS invasion are usually absent. It is nessesary to differentiate primary melanotic tumors of the CNS not only from malignant melanoma metastases, but also from other pigmented CNS tumors such as melanotic meningiomas and melanotic schwannomas. They can be diagnosed by immunohistochemistry and electron microscopy: S-100, vimentin and HMB-45 are positive in meningeal melanocytomas and in schwannomas. In contrast to melanocytomas or schwannomas, meningiomas are EMA-positive. Electron microscopy reveals basal lamina formation around cell groups in melanocytomas, while in schwannomas the basal lamina surrounds every single cell. Melanocytomas contain melanosomes at varying differentiation stages. In contrast to meningiomas, there are no desmosomes [1,5,8,10,13].
The histological diagnosis of melanocytoma is no guarantee for a good clinical outcome , because meningeal melanocytomas can take an aggressive or repetitive course according to some case reports [1, 3,6,7,9]. The correct diagnosis of melanocytic CNS tumors is a diagnostic challenge. Primary melanocytic CNS tumors have a highly variable biological behaviour, their clinical course ranging from benign to highly malignant with the meningeal melanocytoma at one end and the malignant melanoma at the other end of the spectrum. In our case it remains elusive whether a transition has taken place, which has not become morphologically apparent.
There are reports of leptomeningeal melanomas with long-term survival and even permanent cure. Metastases of melanomas often show extensive necrosis, marked cell polymorphy and numerous mitoses. The differentiation between a low-grade melanocytoma and a high-grade melanoma cannot always be made histologically. An attempt to differentiate the extremes morphologically was published by Brat et al . A study with an adequate number of cases and close clinical follow up would be necessary to gain a better understanding of the nature of the tumor .
The biological behavior of this rare tumor might become more predictive by analyzing the distribution of progression-associated melanotic antigens (PAMA) such as VLA-2, ICAM-1, HLA-DR, vitronectin receptor, and transferrin receptor.
Contributed by Monika Koenigsmann, Günther Jautzke, Matthias Unger, Agota Théallier-Janko, Thomas Wiegel, Gisela Stoltenburg-Didinger