Cytologic and ultrastructural studies of a rare breast carcinoma with osteoclast‐like giant cells

: An unusual case of breast carcinoma with osteoclast-like giant cells (OGCs) was reported, for which the diagnosis was made by aspiration biopsy. The OGCs appeared to derive from large mononuclear cells, probably via their cellular fusion. The mononuclear cells had abundant lysosome-like granules, endoplasmic reticulum, and mitochondria. They were thought to be histiocytic cells, though active phagocytosis could not be demonstrated. Histologically, the tumor showed a pattern of well differentiated ductal adenocarcinoma, of which the stroma was crowded with histiocytic cells, OGCs, and lymphocytes. Marked hemorrhage and fibrosis were also seen. The tumor cells tended to be distorted and greatly diminished in accord with the accumulation of OGCs.