Pleomorphic Lipoma : Rb1
Cytogenetics: Both spindle cell and pleomorphic lipomas show rearrangements of 16q or 13q in most cases. Deletion of 13q14 region causes loss of tumor suppressor gene RB1, encoding the Rb protein. The same deletion is also found in cellular angiofibroma and mammary-type fibroblastoma. Rb immunohistochemistry has proved to be a useful tool in separating spindle cell and pleomorphic lipomas (no Rb expression) from its histologic mimics (intact Rb expression). In this image, the spindle cells and large multinucleated giant cells are Rb negative (lower half of the image). The staining within inflammatory cells in the stroma and endothelial cells in blood vessels serves as a nice internal positive control.