These tumors in dogs can metastasize widely and have a predilection for flat bone, especially the spine, pelvis, ribs, skull, and proximal extremities. They have been reported less commonly as primary tumors (extramedullary) in long bones as well as the vertebrae, mandible, larynx, tongue, penis, rectum, cranium, spleen, liver and stomach.
Plasmacytomas have a preponderance for overproduction of Bence-Jones proteins, an homogenous immunoglobulin (IgG, IgA or IgM) which can mimic normal immunoglobulins, resulting in a monoclonal gammopathy on serum electrophoresis. Some rare plasmacytomas are nonsecretory but the majority are secretory, leading to hyperviscosity syndrome.
The cause(s) of plasmacytoma are not clearly established, but genetics, viral infections, chronic immune stimulation due to multiple parasites, and exposure to carcinogens have been identified as possible contributing factors.
Affected dogs can exhibit signs of lethargy, weakness, lameness, bone pain, hemorrhage (e.g, petechiae on mucous membranes, gingival bleeding, and epistaxis), polyuria / polydypsia, and/or neurologic deficits. Other presenting signs of disease may include hypertension, ophthalmic abnormalities (e.g., venous dialation with sacculation, retinal hemorrhages, and retinal detachment), neurologic dysfunction (including seizures), organomegaly, and suggestion of multiple organ failure.
Clinical signs and symptoms may be present for up to a year before a definitive diagnosis of multiple myeloma is made.
Diagnosis of plasmacytoma in dogs can be difficult, but a number of satellite indices are suggestive, including radiographic evidence of osteolysis, demonstration of greater than 20% plasma cells in bone marrow biopsies, a monoclonal gammopathy on serum protein electrophoresis or the presence of Bence-Jones proteinuria.
With bone marrow biopsy, the presence of large aggregates or sheets of plasma cells within the marrow it is highly suggestive of multiple myeloma, if not diagnostic. Secondary amyloidosis in affected organs has been reported.
Chemotherapy is the definitive treatment with over 90% of dogs treated with chemotherapy exhibit clinical improvement. However, eventual relapses are common 1 - 2 years later.
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