October 2019




Signalment (JPC # 2731574): Age and breed unspecified dog


HISTORY: Solitary dermal mass protruding from the lip


HISTOPATHOLOGIC DESCRIPTION: Haired skin, lip (per contributor): Expanding the dermis, compressing surrounding dermal collagen and adnexa, elevating the overlying minimally hyperplastic and hyperkeratotic epidermis, and extending to the deep border is an unencapsulated, well demarcated, densely cellular neoplasm which is separated from the overlying epidermis by a narrow zone of dermal collagen devoid of neoplastic cells (Grenz zone). The neoplasm is composed of round cells arranged in sheets and indistinct nests and packets on a preexisting collagenous stroma. Neoplastic cells have variably distinct cell borders and moderate amounts of eosinophilic, granular cytoplasm. Occasional cells, often toward the periphery of the neoplasm, have a perinuclear pale area (Golgi zone, hof). Nuclei are irregular with finely stippled to dense chromatin with one variably prominent nucleolus. Anisocytosis and anisokaryosis is moderate. Mitotic figures average 1 per HPF. There are few scattered multinucleate neoplastic cells with up to 5 nuclei, occasional single cell necrosis, and few foci of hemorrhage, fibrin, and edema. Within the superficial dermis there is minimal pigmentary incontinence and few scattered lymphocytes.


MORPHOLOGIC DIAGNOSIS: Haired skin: Plasmacytoma, breed unspecified, canine


Signalment (JPC #4076145): 12yo F/S Husky.


HISTORY: Mass, right mandible near lip


HISTOPATHOLOGIC DESCRIPTION: Haired skin, lip (per contributor): Expanding the dermis, compressing adnexa, elevating the epidermis, and extending into the subcutis and to surgical margins is an unencapsulated, well-demarcated, poorly circumscribed, moderately cellular neoplasm composed of round cells arranged in sheets on a preexisting collagenous stroma, surrounded and separated by lakes of amorphous, smudgy, extracellular, eosinophilic material (amyloid). Neoplastic cells have distinct cell borders, a moderate amount of eosinophilic cytoplasm which often contains a perinuclear pale area (Golgi zone, hof), and a round to irregular nucleus with coarsely stippled chromatin and 1-3 variably prominent nucleoli. Anisocytosis and anisokaryosis are moderate; mitoses average 3 per 10 40x HPF. There are occasional multinucleated neoplastic cells with up to 3 nuclei, and rare intranuclear cytoplasmic invagination. Within the neoplasm are multifocal infiltrates of lymphocytes and areas of hemorrhage, fibrin, and edema. The neoplasm is separated from the epidermis by a broad, up to 200um wide, dermal band devoid of neoplastic cells (Grenz zone). Within and surrounding the lakes of amyloid are infiltrates of epithelioid macrophages and multinucleated giant cells with up to 27 nuclei, both Langhans type with peripheral nuclei and foreign body type with central nuclei.


SLIDE C (Congo red): Lakes of eosinophilic, amorphous extracellular material are diffusely orange-red (congophilic), and exhibit apple-green birefringence under polarized light (amyloid).


SLIDE D (MUM1): Within the external control (canine lymph node) there is moderate, multifocal, intranuclear immunoreactivity of paracortical and medullary plasma cells. Within the section of interest, there are occasional non-neoplastic plasma cells within the dermis with moderate intranuclear immunoreactivity (internal control). Neoplastic cells demonstrate diffuse, strong intranuclear immunoreactivity.


MORPHOLOGIC DIAGNOSIS: Haired skin, lip: Plasmacytoma with amyloid, husky, canine.


SYNONYMS: Cutaneous extramedullary plasmacytoma (EMP); Plasma cell tumor



·       Cutaneous extramedullary plasmactyomas occur commonly in the dermis of middle aged or older dogs, and are rare in cats

·       Originate from variably differentiated plasma cells

·       Plasmacytic tumors:

·       Extramedullary plasmacytomas (D-N10): Solitary or rarely multiple tumors arising in soft tissues

·       Solitary osseous plasmacytoma: Single tumor arising in bone

·       Multiple myeloma: Involving bone marrow of multiple bones, plasma cells comprise over 30% of marrow, monoclonal gammopathy, Bence-Jones proteinuria, radiographic evidence of osteolysis

·       Extramedullary plasmactyoma, especially in the oral cavity and the skin, tend to be benign; whereas, osseous lesions tend to progress to systemic multiple myeloma

·       Extramedullary plasmactyoma of the esophagus, stomach, intestine, and rectal area tend to be more aggressive lesions that metastasize to regional lymph nodes

·       Rarely, cutaneous plasmacytomas may precede or be present simultaneously with multiple myeloma

·       Attempts have been made to develop grading systems, but are of little prognostic value



·       Unknown

·       Amyloid is a pathogenic proteinaceous substance composed of polypeptides arranged in beta-pleated sheets; amyloid associated with plasma cell tumors is AL, amyloid light chain; derived from plasma cells; contains immunoglobulin light chains (either λ light chains or κ light chains)



·       Average age of dogs is 10 years with no sex predilection; Cocker Spaniels may be predisposed

·       Commonly present on digits, around the mouth (lips and chin), and ears

·       Solitary; may be polypoid when associated with the ear canal; rarely multiple

·       Tumors of digits are often ulcerated and hemorrhagic

·       Surgical excision is curative; local recurrence and metastases are rare



·       Tumors are usually 1 to 2 cm, raised, smooth, well circumscribed, firm to soft, and pink to red



·       Round cells arranged in sheets

·       Cells are either well differentiated or atypical and pleomorphic

·       Some cells may retain a Golgi zone or perinuclear hof

·       Nuclei are round or crescentic (helmet cells), often eccentrically placed and vesiculate,with course chromatin forming a “clockface” pattern or finely stippled/hyperchromatic

·       Approximately 10% may contain amyloid, more frequently found in feline EMP, which is often accompanied by granulomatous inflammation; occasionally, multifocal chondrous metaplasia can develop in the deposited amyloid

·       Intravascular neoplastic cells have been reported in a small subset of cases, but are not indicative of biological behavior (Ehrensing; J Vet Diagn Invest; 2018)

·       Occasionally can take on a “pseudoglandular” appearance (McHale; J Vet Diagn Invest; 2018)



·       Oval to round, eccentric nucleus with marginated chromatin described as “cartwheel” or “clockface”

·       Ample cytoplasm with abundant rough endoplasmic reticulum and a well developed perinuclear Golgi apparatus



·       Immunohistochemistry: Plasma cells are immunoreactive for: vimentin, CD79a, CD20, CD18, immunoglobulin kappa light chains, MUM1/IRF4

·       Histiocytomas can also be immunoreactive for MUM1/IRF4; a recent study showed strong nuclear immunoreactivity in all 20 tested canine cutaneous histiocytomas (Stilwel; Vet Pathol; 2018)

·       Histochemistry:

·       Amyloid stains with Congo red (congophilic)

·       Most amyloid stains with thioflavine-T, a fluorescent dye

·       Methyl green pyronine stains cytoplasmic ribonucleic acid magenta



·       Gross: Similar to canine cutaneous histiocytoma (I-N22)

·       Microscopic:

·       Amelanotic epithelioid melanoma (I-N24): Finely granular, pale cytoplasm; prominent nucleoli; junctional activity; and positive Fontana-Masson, Melan-A, and/or S-100 stains

·       Histiocytoma (I-N22): Cords of cells perpendicular to skin surface, lymphocytic infiltrates, no tumor giant cells

·       Mast cell tumor (I-N21): Granular cytoplasm, stromal collagen, positive toluidine blue and Giemsa, granules stain orange-brown with methyl green pyronine

·       Cutaneous lymphoma (I-N25, I-N26): Rare, most often multiple, recur after surgical excision



·       Although rare, they have been reported in aged cats; occur in the skin, gastrointestinal tract, retroperitoneal space, upper lip, gingival, orbit, respiratory tract, and cerebrum (Sykes; J Comp Pathol; 2017)

·       One report in sheep

·       Two cases of extramedullary plasmacytoma in the salivary gland of 2 Syrian hamsters

·       One report of an intracerebral plasmacytoma in a mule deer



1.     Clancy CS, Roug A, Armien AG, Van Wettere AJ. Intracerebral malignant plasmacytoma in a mule deer. J Comp Pathol. 2016;154(2-3):268-71.

2.     Ehrensing G, Craig LE. Intravascular neoplastic cells in canine cutaneous plasmacytomas. J Vet Diagn Invest. 2018;30(2):329-332.

3.     Kumar V, Abbas AK, Aster JC, eds. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Philadelphia, PA: Elsevier Saunders; 2015:256-262.

4.     Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed, St. Louis, MO: Elsevier; 2016:735.

5.     McHale B, Blas-Machado U, Oliveira FN, Rissi Dr. A divergent pseudoglandular configuration of cutaneous plasmacytoma in dogs. J Vet Diagn Invest. 2018;30(2):260-262.

6.     Stilwell JM and Rissi DR. Immunohistochemical labeling of multiple myeloma oncogene I/interferon regulatory factor 4 (MUMI/IRF-4) in canine cutaneous histiocytoma. Vet Pathol. 2018;55(4):517-520.

7.     Sykes SE, Byfield V, Sullivan L, Bender SJ, Moore PF, Sanchez MD. Feline respiratory extramedullary plasmacytoma with lymph node metastasis and intrahistiocytic amyloid. J Comp Pathol. 2017;156(2-3):173-177.

8.     Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016:27-28.

9.     Valli VEO, Kiupel M, Bienzle D. Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier; 2016: 226-228.


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