[Melanocytic schwannoma. Light-and electron microscopic findings on the morphology, diagnosis, and differential...

A melanocytic schwannoma was extirpated from the posterior mediastinum of a ten-year old girl The tumor consisted of cells which were spindle-shaped to polygonal in shape. Spindle-shaped cells were arranged in short, somewhat curved fascicles. In some areas a strand-like arrangement of cells gave rise to an epithelioid appearance. Typical features of benign schwannomas such as nuclear palisading and myxoid texture were absent. The essential histologic hallmarks were melanin deposition and the presence of small psammoma body-like, calcified spherules. Electron microscopic examination confirmed the schwannian differentiation of the tumor cells. These features included interlocking cellular processes, along with a tendency to display the wrap-around phenomenon and a sometimes multi-layered lamina externa. Intracytoplasmic melanin synthesis was evidenced by demonstration of melanosomes in different phases of maturation. The differential diagnosis is discussed with particular emphasis on the need to distinguish this tumor from malignant melanoma. Melanocytic schwannomas should be regarded as potentially malignant. The tendency and degree of invasiveness, mitotic frequency and the ability of tumor cells to form lamina externa have the greatest prognostic value.