Plasmacytomas are solitary collections of cancerous plasma cells. Normal plasma cells are immune system cells that are responsible for the production of many different types of antibodies that fight infection. The proliferation of cancerous plasma cells results in a population of plasma cell clones that may produce a high number of one specific types of antibody in the blood (monoclonal gammopathy). Plasmacytomas can originate in bone or soft tissue and are referred to as solitary osseous plasmacytoma (SOP) or extramedullary plasmacytoma (EMP). When a diagnosis of any plasmacytoma has been made, the patient should be staged, making sure that it is confined to a single location in the body.
Extramedullary plasmacytoma (EMP)
EMPs are typically classified according to location: cutaneous/oral and noncutaneous. Cutaneous plasmacytoma is a tumor of older dogs with German Shepherds being somewhat over-represented. Tumors can cover the trunk, limbs, head (especially the ears), and oral cavity. For the most part, canine cutaneous plasmacytomas are benign, carrying an excellent prognosis following complete surgical excision. In a study of dogs with plasmacytomas involving a large number of cases, <4% recurred following surgical excision, <2% spread to other sites, and only 1% of dogs developed a systemic condition called multiple myeloma (see below). Radiation therapy is effective for cases in which surgical resection is not an option due to tumor size or location. Chemotherapy has been successful in cases where spread to other locations occurred. The biologic behavior of noncutaneous EMP tends to be more aggressive. Tumors have been reported to occur in the esophagus, stomach, and small and large intestines. Metastasis to the regional lymph nodes is common, warranting chemotherapy. The prognosis for plasmacytomas in the cat is unknown due to the paucity of information available on this tumor in the veterinary literature concerning felines.
Solitary osseous plasmacytoma (SOP)
SOPs can affect virtually any bone in the body. Local therapy is the treatment of choice once adequate staging has been performed to rule out systemic involvement. Surgical intervention is warranted in any case where the tumor has resulted in an unstable long-bone fracture or neurologic complications from spinal compression from an SOP in the spinal column. Surgical removal alone or in combination with radiation therapy has been successful in the local control of SOPs. Unfortunately, most patients with an SOP at the time of diagnosis will go on to develop a systemic disease, called multiple myeloma. Multiple myelomas is a systemic proliferation of plasma cells that typically involves multiple sites in the bone marrow and other organs. The plasma cells may cause an elevation in protein levels known as a monoclonal gammopathy—an overabundance of a single antibody that circulates in the blood and can spill into the urine. Given this information, there is some controversy in veterinary medicine as to whether chemotherapy should be initiated at the time of diagnosis of an SOP. Extrapolating from what we know in human medicine, adding chemotherapy at initial diagnosis provides no significant survival advantage. Unless the tumor is high grade, careful follow up is generally recommended to closely monitor blood work, radiographs, and the clinical symptoms of the patient.