A previously healthy 39-year-old man presented with progressive abdominal pain over one week. His past medical history was significant for B-lymphoblastic leukemia, diagnosed when he was 16 years old. He received a 2.5-year course of chemotherapy and has been in complete remission since that time.

He had a CT scan to evaluate these symptoms and was noted to have mesenteric and retroperitoneal lymphadenopathy. A PET-CT showed extensive FDG-avid mesenteric upper abdominal and retroperitoneal adenopathy (Fig. 1A). He subsequently underwent a percutaneous biopsy of one of the mesenteric lymph nodes which was diagnostic of histiocytic sarcoma.

Learning points:

1. Histiocytic sarcoma is an extremely rare malignant neoplasm of presumed myeloid origin that shows morphologic and immunophenotypic characteristics of mature macrophages/histiocytes.
2. A subset of these neoplasms can arise through clonal evolution of pre-existing hematologic neoplasms such as follicular lymphoma, chronic lymphocytic leukemia/small lymphocytic lymphoma, B-lymphoblastic leukemia (possible in this case) and others.
3. They exhibit variable outcome with no standardized therapy.
4. Myeloid sarcoma needs to be considered and excluded in these cases since this is a close differential diagnostic possibility.