Cutaneous plasmocytoma C90.0

Mac Intyre, 1850; Kahler, 1889

Plasmocytoma is the solitary organ manifestation of "multiple myeloma". Plasmocytomas of the skin are very rare. They can occur as:

• Primary cutaneous (extramedullary) B-cell NHL with a low degree of malignancy (see below Primary cutaneous B-cell lymphomas) without (primary detectable) systemic involvement.

or as

• Secondary (metastatic) manifestations of multiple myeloma (these are somewhat more common). In this case, they would be referred to as cutaneous organ manifestations of multiple myeloma . Metastatic cutaneous infiltrations (see also leukemia cutis) usually occur in scarred structures (e.g., sternotomy scars; Li A et al. ). They are associated with a significantly worse prognosis than primary cutaneous plasmacytomas(Yoo J et al.).

Unknown; extramedullary plasmocytomas are frequently found after organ transplants. Individual reports exist about an EBV association.

The average age is 70 years

Nodular or diffuse tumour cell aggregates in the entire dermis and possibly in subcutaneous fatty tissue. Mostly atypical plasmacytoid cells, mature or immature plasma cells; more rarely immunoblasts. No epidermotropy. Intracellular: immunoglobulins, often monoclonal IgA synthesis and more rarely IgG synthesis. In PAS preparations, PAS-positive inclusions in the cytoplasm (Russel corpuscles).

Immunohistology: Tumor cells are pos. for CD79a, CD38, CD138 and neg. for CD20 and CD45. The analysis of the IgH gene shows mostly monoclonality.

Histological and immunohistological clarification of the findings is important, as well as the exclusion of systemic involvement (extremely accelerated BSG, 1-h value:>100mm n.W., so-called fall reduction, proteinuria with L-chain excretion = (Bence-Jones proteins; Bence-Jones proteins are found in about 60% of all multiple myelomas from IgG or IgA light-chain myeloma). Low-dose CT is recommended as the most sensitive imaging method. Skeletal scintigraphy is less suitable because myeloma foci often do not store data. MRI and PET are suitable for extramedullary foci.

Clinical differential diagnoses: S.u. lymphoma, cutaneous B-cell lymphoma

Histologic Differential Diagnoses:

• Benign plasma cell proliferation
• cutaneous inflammatory pseudotumor.

The term plasmacytoma is only valid for solitary monorganic involvement of an organ, in the absence of bone marrow infiltration.

Low-malignant B-cell NHL, as a systemic disease with mostly multilocular or diffuse infiltration of the bone marrow by malignant cloned plasma cells, is referred to as"multiple myeloma" (C90.00).

Only in case of systemic involvement (multiple myeloma) the following parameters become diagnostically relevant: massive elevation of ESR, paraproteinemia, paraproteinuria (Bence-Jones sample), also cryoglobulins. X-ray skull: moth-eaten osteolysis (note: myeloma cells have no osteolytic activity of their own; however, they stimulate osteolysis via various cytokines (RANKL). (Note: Myeloma cells have no osteolytic activity of their own; however, they stimulate osteoclast activity via various cytokines (RANKL =receptor activator of NF-kB ligand, MIP-1= macrophage inflammatory protein = CCL3) and at the same time inhibit osteoblast activity.

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11. Yoo J et al (2017) Cutaneous plasmacytoma: metastasis of multiple myeloma at the fracture site.
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