mast cell neoplasm mastocytoma A localized tumor composed of sheets of mast cells without atypia. It includes the cutaneous mastocytoma which involves the dermis and subcutaneous tissue, and the extracutaneous mastocytoma. Most cases of extracutaneous mastocytoma have been reported in the lung. SCTID:404171008 UMLS:C0024897 MEDGEN:6237 MONDO:0003079 GARD:0023358 mastocytoma ICDO:9740/1 NCIT:C9303 ICD9:238.5 MESH:D034801