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.
