Histiocytic sarcoma with secondary involvement of the skin and expression of CD1a: evidence of indeterminate cell differentiation?

Background: Histiocytic sarcoma is an exceedingly rare malignant neoplasm composed of cells with a monocyte/macrophage phenotype. In the current nosology of histiocytic neoplasms, histiocytic sarcoma is separate from indeterminate cell histiocytosis, a generally benign disorder characterized by proliferation of a CD1a+ and S-100+ population of cells lacking Birbeck granules usually limited to the skin.

Methods: We present a case of histiocytic sarcoma in a 64-year-old man presenting as a peritonsillar mass and secondarily involving the skin.