Answer: Extramedullary myeloid tumor

Histology: Sections of ileum show transmural involvement by a dense infiltrate of medium to large cells. The cells have moderately pleomorphic nuclei with frequent mitoses. Immunostains show reactivity for CD34, CD43, CD117 (c-kit), CD68 and myeloperoxidase (myeloperoxidase stain shown below).

Discussion: Extramedullary myeloid tumor (aka granulocytic sarcoma, chloroma) is a myeloid leukemia that presents as a discrete mass outside of the marrow. Some of the more common sites of involvement include skull, paranasal sinuses, vertebrae, lymph nodes and skin. Gastrointestinal involvement is less common but is well-described. The most common clinical scenario is a patient with known acute or chronic myeloid leukemia although it may occur in the absence of an antecedent history and without clear marrow involvement. The tumors commonly express blast markers (CD34, CD117) in addition to CD43 and myeloperoxidase. CD68 indicates monocytic differentiation. CD45-LCA is negative in a substantial proportion of cases.