Primary Cutaneous True Histiocytic Sarcoma : A Case with an Indolent Course

True histiocytic sarcoma (THS) is a rare form of malignant neoplasm, which is thought to have a poor prognosis. We report the case of a 47-year-old woman who presented with a solitary cutaneous tumor on the forehead. After excision of the tumor, she had no recurrence or metastasis for 10 years. Microscopic observation showed massive infiltration of histiocytoid tumor cells from the dermis without epidermotropism to the deep muscle layer. The large histiocytoid tumor cells contained folded nuclei with frequent mitoses and abundant cytoplasm, but showed no phagocytosis. Muscle fibers were destroyed by infiltration of the tumor cells. Immunohistochemically, the tumor cells were positive for histiocytic markers (CD68 and lysozyme), but negative for lymphoid and epithelial markers. No rearrangement of the immunoglobulin heavy chain and T-cell receptor β/γ chain genes was demonstrated. Therefore, the lesion was diagnosed as primary cutaneous true histiocytic sarcoma (PCTHS) based on the current, strict definition of histiocytic malignancies. In reviewing the literature, we found that some PCTHSs had an indolent clinical course; therefore, we propose that PCTHSs may have variable prognoses.