Abstract
Histiocytic sarcoma (HS) is a rare cancerous tumor that originates from fully developed histiocytes. It is most often identified by the presence of certain proteins such as the cluster of differentiation (CD) 68, CD163, or lysozyme. HS has been recorded in different sites outside of the lymph nodes such as the gastrointestinal tract, nasal cavities, skin, and bone marrow. Because HS shares similar clinical features with other forms of malignant diseases, diagnosing it becomes incredibly challenging. We report a case of a 40-year-old female who presented with a breast mass for one year. A preliminary diagnosis of a phyllodes tumor was made. However, the morphology along with the immunophenotype picture was diagnostic for HS. Microscopic features showed a well-defined neoplastic growth arranged in sheets and fascicles. Diffuse immunoreactivity was seen for CD45, CD4, CD68, CD163, and vimentin. We present the important histopathological and immunohistochemical characteristics of the tumor in this case.