Answer: Mast cell disease

Histology: Mastocytosis (mast cell disease) is a neoplastic proliferation of mast cells in the skeleton and in the parenchymal organs. Patients frequently present with symptoms of marrow replacement. Histologically, mastocytosis shows sheets of rounded mast cells with centrally located nuclei and amphophilic cytoplasm. The diagnosis of mast cell proliferation can be confirmed with a positive giemsa stain. However, a more specific stain is the immunostain for mast cell tryptase.

Discussion: The lesion is not metastatic carcinoma. Although older men with disseminated radiodense lesions should be considered to have metastatic prostate carcinoma, the nature of the infiltrate in this case is non-epithelial. Immunostains for keratins and epithelial membrane antigen are negative.

The diagnosis is not multiple myeloma. Although multiple myeloma may have a sclerotic presentation in the skeleton, the features of plasma cells are not present in this case. Also, a CD138 stain for plasma cells would be negative.

Although lymphoma may present with radiodense lesions throughout the skeleton, the lymphoid markers would be positive. These markers are negative in mastocytosis.