CD117, but not lysozyme, is positive in cutaneous plasmacytoma.

CONTEXT

CD117 (c-Kit) and lysozyme are frequently expressed by myeloblasts and are sensitive markers for the diagnosis of extramedullary myeloid tumor. The diagnosis of cutaneous plasmacytoma presents a degree of difficulty, particularly with the plasmablastic variant, which can mimic hematologic as well as epithelioid malignancies. Approximately 25% of multiple myelomas express CD117 in the bone marrow by flow cytometry. Lysozyme immunoreactivity has been previously shown in 30% of poorly differentiated myelomas, while it is nonreactive in nonmalignant plasma cells.

OBJECTIVE

To ascertain whether CD117 and lysozyme can aid in the diagnosis of cutaneous plasmacytomas, particularly the plasmablastic type.

DESIGN

Pathology reports of 2357 patients with a diagnosis of multiple myeloma were reviewed to find 13 cutaneous plasmacytomas (8 Bartl grade II, 5 Bartl grade III). Formalin-fixed, paraffin-embedded tissue sections were stained with CD117 and lysozyme on the Dako Autostainer system.Setting.-Patients with the diagnosis of multiple myeloma who developed cutaneous plasmacytoma(s).

RESULTS

The cutaneous plasmacytomas uniformly expressed CD117 in a cytoplasmic or membranous and cytoplasmic distribution with varying degrees of staining intensity unrelated to the Bartl grade of the lesion, while they were uniformly negative for lysozyme.

CONCLUSIONS

CD117 is a sensitive marker for malignant plasma cells in paraffin-embedded tissue, while lysozyme does not help identify poorly differentiated malignant plasma cells. While CD117 alone does not distinguish extramedullary myeloid tumor from poorly differentiated myeloma, the combination of CD117 and lysozyme may allow their differentiation. The possibility of c-kit inhibitors being used in the treatment of other hematopoietic malignancies allows speculation regarding implications for the treatment of multiple myeloma.