In the previous article, we focused on a diffuse type of plasma cell tumor—multiple myeloma. Another important form of neoplastic plasma cells are plasmacytomas, which arise from soft tissue, where they are known as extramedullary plasmacytomas, or from bone, where they are known as solitary osseous plasmacytomas.

SIGNALMENT AND PATHOLOGY

EMPs

In a large study evaluating 751 canine EMPs, the most frequent sites of origin were the skin (86% of plasmacytomas), mucous membranes of the oral cavity and lips (9%), and rectum and colon (4%). Other sites, including the stomach, small intestine, spleen, genitalia, and eye, represented 1% of plasmacytomas.³ Other reports suggest that oral plasmacytomas are more common and that about one-fourth of EMPs in dogs are within the oral cavity.^4-6 Frequently reported locations for cutaneous tumors include the trunk, limbs, and head, with the ear being the most common site on the head.^1,4,6 The true incidence of EMPs may be underestimated because of previous classification as reticulum cell sarcoma, neuroendocrine tumor, poorly differentiated round cell tumor, or cutaneous lymphoma.^6,7

The incidence of EMP in cats is low, and few case reports exist. Affected cats are usually older, with a mean age of 8.5 years. Skin is the most common site affected, but other reported sites include the oral cavity, gastrointestinal tract, retroperitoneum, brain, and orbit.^8-10

Antigenic stimulation, such as that resulting from gingivitis and periodontal disease, has been speculated to be a factor in the development of oral EMP. This hypothesis is supported by the identification of mature T cells and dendritic cells within oral EMP, suggesting both neoplastic and inflammatory components.²

SOPs

Solitary osseous plasmacytoma (SOP) is rarely reported in dogs and cats, and most cases progress to multiple myeloma months to years after local tumor development. Reported sites include the vertebrae, zygomatic arch, and rib.^11-13 Anecdotally, we have also seen cases of SOP in which long bones are affected. In people, SOPs are most often found in the axial skeleton, usually the vertebrae and skull.¹⁴

CLINICAL SIGNS

EMPs

Clinical signs, other than the visible tumor mass or oral bleeding, are uncommon in most dogs with localized mucocutaneous EMP.^2,6 Tenesmus, rectal prolapse, hematochezia, and rectal bleeding are the most frequent clinical signs in dogs with colorectal plasmacytomas.^3,6 In case reports, EMPs of the larynx and trachea are associated with dysphonia and dyspnea, respectively, and intracranial EMP is associated with central nervous system signs.^7,15,16 Globulin-secreting EMPs have been reported, and in these rare cases, clinical signs consistent with hyperviscosity syndrome may be present.^4,17,18

Figure 1. An oral EMP discovered on physical examination in a boxer. (Photograph courtesy of Dr. Laura Garrett.)

SOPs

Patients with SOP may present with bone pain, lameness, pathologic fracture, or neurologic deficits, depending on the site affected. Paresis or paralysis may be present with vertebral lesions and spinal cord compression.^11,14