Chondrolipoma of the breast as a rare variant of myofibroblastoma: an immunohistochemical study of two cases.

Chondrolipoma of the breast is a very rare tumor whose histogenesis remains obscure. We report two cases (56-year-old and 43-year-old women) and present the results of an immunohistochemical study which strongly suggests that this tumor is a variant of myofibroblastoma. The tumors predominantly consisted of lipoma-like, mature adipose tissue, and many islands of hyaline cartilage. A proliferation of spindle cells associated with the deposition of collagen fibers was also seen. On immunohistochemical examination, spindle cells showed cytoplasmic reactivity for vimentin, desmin, bcl-2, and α-smooth muscle actin, as well as nuclear reactivity for estrogen receptor (ER) and progesterone receptor (PgR). Chondrocytes were immunoreactive for ER, PgR, S-100 protein, and Sox9. The nuclei of adipocytes, chondrocytes, and spindle cells were not immunoreactive for Rb (retinoblastoma) protein. The immunoreactivity of spindle cells for muscle markers indicates myofibroblastic differentiation, and the lack of the nuclear expression of Rb protein suggests the close relationship of this tumor with myofibroblastoma and spindle cell lipoma. The immunoreactivity of chondrocytes for ER and PgR suggests that they are derived from metaplasia of hormone-sensitive spindle cells. These findings support the concept that chondrolipoma of the breast could be a lipomatous variant of myofibroblastoma associated with cartilaginous metaplasia and that it should be added to members of the "13q/Rb family of tumors."