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Feline extramedullary plasmacytoma (EMP)

©M. Majzoub, W. Breuer, S. J. Platz, R. P. Linke, W. Linke and W. Hermanns. Histopathologic and Immunophenotypic Characterization of Extramedullary Plasmacytomas in Nine Cats. Vet Pathol 40:249-253 (2003). http://www.vetpathology.org/cgi/content/full/40/3/249

Plasmacytomas are neoplasms consisting mainly of plasma cells at varying stages of differentiation.11 The solitary myeloma and multiple myeloma are additional types of plasma-cell–derived tumour. The latter are primarily located in the bone marrow. Extramedullary plasmacytomas (EMPs) are characterized by their extraskeletal location.8,29 As defined by the World Health Organization, EMPs consist of atypical neoplastic plasma cells with monoclonal expression of immunoglobulin (Ig) light chains or heavy chains.15 They differ from other non-Hodgkin lymphomas by their predominantly post-mitotic and terminally differentiated plasma cells. The main cellular function, production and secretion of Igs, is largely unaffected.22 Whereas the EMP is a frequent neoplasm in dogs,4,13,16,23,25–27 it is a sporadic tumour in man.3,7,29,30 Judging from the existing case reports involving only one or two animals, the incidence of EMP in cats seems to be low. Feline EMPs have been described as occurring in the skin,4,5,13,19 gastrointestinal tract,24,31 retroperitoneal space,14 upper lip, gingiva,9 and orbita.28  Affected cats were generally more than 10 years old; the average age of the animals was 10.6 years.13,19 A male predisposition could be clearly concluded from the literature (male to female ratio was 2:1). The European Short-Haired cat was the most commonly affected. One case each was reported in the Abyssinian,28 the Long-Haired, the Persian,9 and the Burmese13 breeds. Tumour-associated monoclonal gammopathy was reported in three cases.5,14,28

The literature is uniform in describing the morphology as a monomorphic population of plasma cells with isolated bi- or multinucleated giant cells.4,28,31  In several cases, immunohistochemistry revealed monoclonal expression of λ-Ig light chains,4,6,9,19 whereas λ-Ig light-chain expression was found in only one case.9 Ig heavy chains of the immunoglobulin G (IgG)9 and immunoglobulin (IgA)31 types also have been confirmed in the tumour cells. Amyloid deposits in feline EMPs were demonstrated by several authors,4,5,13,14,19,24 although the amyloid was identified as AL λ-amyloid in only three cats.19,24 To exclude a multiple myeloma, some authors examined the bone marrow for tumour cell infiltrates in the cats.5,10,14,31

In humans, the extramedullary plasmacytoma is uncommon and accounts for less than 5% of all plasmacellular neoplasia.7,29,30 Although EMPs are commonly found in dogs, mostly in the form of skin and mucosal neoplasms,20 they seem to be rare in cats. The latter conclusion can be drawn both from the fact that so far only single-case reports exist in the literature and from the incidence of cutaneous EMPs, as estimated from the number of biopsies submitted to this department for histologic examination. The incidence of EMPs in other parts of the body besides the skin remains unknown, although, according to other authors, it can be assumed to be low.4,5,13 From our own series, it also can be confirmed that EMPs in cats are mainly cutaneous neoplasms, as observed previously.4,5,9,13 The occurrence of EMPs in other organs, e.g., digestive tract 24,31 and liver,10 is rare. With an average age of 8.2 years, the EMP occurs predominantly in older cats. This, as well as the preference for the male sex, agrees with the result in the literature.4,5,13

As in dogs, a typing of EMPs based on morphologic criteria also is possible in cats. With the exception of the hyaline type described in dogs, the same types of EMP found in dogs were found in cats. The absent hyaline type might be found in cats, if a larger number of tumours could be examined. Because of the small number of cases and a lack of follow-up studies, no claim can be made currently concerning the diagnostic significance of the typing of feline EMPs.

Immunohistochemistry of the feline EMPs mainly revealed λ light-chain expression. This is very similar to the physiologic distribution of light chains in normal feline plasma cells with primary expression of the λ light chain.1 Monoclonal λ light-chain expression was reported in only one case.9 With respect to the immunohistochemical results in our series, the possibility that the tumour plasma cells could express Ig heavy chains not detectable with the routinely used antibodies cannot be excluded. According to the literature, amyloid is more frequently found in feline EMPs than in canine EMPs. Six out of 12 published cases of EMPs in cats report on amyloid deposits.4,5,14,19,24,28 In the present study, three of the nine EMPs contained amyloid, identified as AL λ-amyloid. This is consistent with the result of the only study published to date of an immunohistochemically investigated feline EMP.24

As reported by other authors,5,10,14 immunohistochemical examination for FeLV-antigen (p27 protein) was negative for all tumours. Communication of the above results may contribute to the already existing knowledge regarding extramedullary plasmacytomas in cats.

Fig. 1. Skin, paraffin section; European Short-Haired cat, 7-year-old spayed female. Feline extramedullary plasmacytoma (EMP, asynchronous type of tumour cells) with basophilic cytoplasm, a clear perinuclear halo, and a blastic nucleus with a central nucleolus. Giemsa. Bar = 20 µm.
Fig. 2. Immunohistochemical demonstration of Ig-light-chains in the cytoplasm of tumour cells. Same cat as in Fig. 1. Bar = 20 µm.
Fig. 3. Feline EMP with homogenous eosinophilic amyloid deposits. HE. Bar = 50 µm.
Fig. 5. Skin; European Short-Haired cat, 13-year-old castrated male. Congo red–staining of EMP, mature type of tumour cells. Bars = 20 µm.
Fig. 6. All Congo-red positive areas within the tumour are identified as amyloid by green-birefringence in polarized light. Same cat as in Fig. 5. Bars = 20 µm.
Fig. 7. Immunohistochemical identification of AL amyloid with the anti-AL-λ antibody ULI. Same cat as in Fig. 1. Bar = 20 µm.
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