Erdheim-Chester disease involving the breast--a rare but important differential diagnosis.

Erdheim-Chester disease (ECD) is a rare non-Langerhans cell histiocytosis characterized by multisystem infiltration by foamy histiocytes surrounded by fibrosis. ECD often involves the long bones, skin, and retroperitoneum, whereas breast involvement is very rare with only 6 reported cases in English literature. We report a case of ECD presenting within the right breast as a clinically malignant tumor, in addition to bilateral sclerotic lesions of the femurs, bilateral soft tissue masses of the cerebellum, and multiple subcutaneous nodules on the abdominal wall in a 61-year-old woman. Histologically, there was a prominent infiltrate of foamy histiocytes with scattered Touton-type giant cells, lymphocytes, and plasma cells. The foamy histiocytes were arranged in small clusters or scattered singly in the background of fibrosis. However, in some areas, there was a prominent proliferation of fibrosis with scant cellular infiltrate including histiocytes. The diagnosis of ECD was made by characteristic histopathologic features in addition to clinical-radiographic features and the typical immunoprofile (positive for cluster of differentiation 68 [CD68], CD163, and p16; negative for CD1a and S-100). Although rare, ECD must be considered in the differential diagnosis of clinically malignant tumor of the breast. To our knowledge, this is the second case of ECD involving the breast in which a valine 600 glutamic acid mutation was detected, which probably represents a clonal disorder of non-Langerhans cells.