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We present a case of an adult male with a solitary mast cell tumor of the skin with unusual nuclear pleomorphism and mitotic activity. The tumor was excised, recurred within 2 years, was reexcised after 4 years and did not recur >6 years after diagnosis. The tumor showed progressive cytonuclear atypia and a high mitotic and proliferation rate by Ki67-staining from the onset. No KIT mutations were identified in the tumor and bone marrow. Serum tryptase levels and a bone marrow aspirate and trephine biopsy were normal. Although the histomorphology of the skin tumor was consistent with mast cell sarcoma, the clinical behavior without systemic progression argued against this diagnosis. The tumor was finally considered as atypical mastocytoma, borderline to mast cell sarcoma. Currently, the patient is in close follow-up and still in complete remission., (© 2021 The Authors. Journal of Cutaneous Pathology published by John Wiley & Sons Ltd.)

An extremely rare solitary mast cell tumor of the lung was studied histologically, immunohistochemically, and ultrastructurally. The histologic features of the tumor included nodular growth of well-differentiated mast cells and clear cells with no granules. The current case is the third case of a solitary mast cell tumor (granuloma) of the lung in the literature. Clinicopathologic features of this tumor are compared with the other two cases reported previously in the international literature, and the nature of the clear cells is discussed.