Malignant histiocytic neoplasm with histiocytic sarcoma phenotype is a
rare malignant neoplasm, distinguished by malignant cells with
phenotypical characteristics of mature tissue histiocytes. Histiocytic
sarcoma typically presents as a primary malignancy, although it can also
present as a secondary malignancy, and is rarely seen in the pediatric
population. Due to the rarity of this condition, diagnosis of
histiocytic sarcoma is difficult and considered a diagnosis of
exclusion. We describe a unique case of a chronic upper eyelid lesion
with biopsy findings of a highly atypical histiocytic neoplasm
concerning for histiocytic sarcoma in a child treated with excision and
observation alone.