“Occult” mastocytosis with activating c‐kit point mutation evolving into systemic mastocytosis associated with plasma cell myeloma and secondary amyloidosis

A case of a 70‐year‐old man presenting with exsudative enteropathy due to light‐chain‐associated amyloidosis is reported. The diagnosis of systemic mastocytosis associated with IgG/λ plasma cell myeloma and secondary generalised amyloidosis was carried out by morphological evaluation of bone marrow biopsy. The c‐kit point mutation D816Y was detected by molecular analysis. Two years before, a cystadenolymphoma of the left parotid gland had been removed. A moderate increase of loosely scattered spindle‐shaped mast cells, a subpopulation of them expressing CD25, an antigen that is not expressed by normal or reactive mast cells, was shown by retrospective analysis carried out on an intraparotideal lymph node. The c‐kit mutation D816Y was shown by the molecular analysis of the lymph node. In summary, the notion that systemic mastocytosis may very rarely be associated with B cell neoplasms and that neoplastic mast cell infiltrates may be obscured because of only a minimal increase of atypical mast cells, which are outnumbered by other non‐neoplastic cells in the same tissue, is supported by this case. This finding was preliminarily termed “occult” mastocytosis.