Extramedullary and solitary osseous plasmacytomas in dogs and cats - Veterinary Medicine
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Extramedullary and solitary osseous plasmacytomas in dogs and cats
Patients with the soft tissue form of this plasma cell tumor have a good prognosis, while those with the form that originates from bone may develop multiple myeloma.



EMP is most often diagnosed by using cytology or immunohistochemistry. Histologic findings of EMP reveal that these tumors are nonencapsulated and locally destructive.5 A histologic classification system has been developed that may be helpful in diagnosing plasmacytomas, but there is no correlation among cell type, biologic behavior, and prognosis.1,5 Localized amyloid deposition may be detected in some tumors, and immunohistochemisty may demonstrate immunoglobulin light chain (lambda or kappa) expression. Lambda light chain monoclonality is observed in most dogs with EMP.1,5,20 The immunohistochemical marker multiple myeloma 1/interferon regulatory factor 4 (MUM1/IRF-4) may also assist in diagnosis.21

Figure 2. A transverse computed tomography image of the oral cavity in a dog with a maxillary EMP. The mass is locally invasive and destructive to the underlying bone (arrow). (Image courtesy of Dr. Laura Garrett.)
It is important to consider thorough staging in dogs and cats before pursuing therapy for EMP and SOP to rule out multiple myeloma and ensure that there is no evidence of systemic illness. Staging is especially important with plasmacytomas that tend to be associated with more aggressive disease, such as SOP and noncutaneous, non-oral EMP. Staging tests include skeletal radiography, bone marrow aspiration, and serum protein electrophoresis. In cases of oral and cutaneous EMP, which tend to be benign, staging is less important, and a minimum database and local lymph node aspiration are likely to be adequate before pursuing treatment. Imaging studies of oral EMP, such as high-detail dental radiography or computed tomography, may improve assessment of tumor infiltration and optimize surgical planning (Figure 2).2



Unlike multiple myeloma, EMP generally carries a favorable prognosis. These tumors tend to be locally invasive but have a low metastatic rate, and complete surgical removal is often curative. In compilations of reports of canine cutaneous and mucocutaneous EMPs, surgical excision was curative in about 90% to 95% of cases. Local recurrence rates of 5% to 8% were encountered most often with microscopically incomplete resection, and distant metastasis was present in < 4% of cases.1,4,6 In cases of incomplete excision, additional surgery, adjunctive radiation therapy, or systemic chemotherapy may extend the disease-free interval.2,4 Multiple cutaneous EMP that is aggressive in behavior has been reported in dogs.22 Additionally, progression of EMP to multiple myeloma in cats and dogs has been reported.23,24

Figure 3. An ultrasonogram of a metastatic abdominal lymph node in a dog with an intestinal EMP. (Image courtesy of Dr. Laura Garrett.)
Noncutaneous, non-oral EMPs are reported to have a more aggressive behavior in dogs (Figure 3). However, dogs treated with surgery alone or a combination of surgery and adjuvant therapy can have extended survival times.3,18,25 In a report of nine dogs treated surgically for colorectal plasmacytomas, the median survival time was 15 months. In two cases in which surgical margins were incomplete or close, the tumor recurred locally five and eight months after resection (Figure 4).3 Adjuvant chemotherapy is recommended if metastatic disease is detected or if incomplete surgical margins are obtained. Systemic chemotherapy has been used after the excision of a globulin-secreting hepatic plasma cell tumor in a dog.18

Figure 4. A colonoscopic image showing a colonic EMP in a dog. (Photograph courtesy of Dr. Laura Garrett.)
Few reports exist regarding cats with EMP. In a study of cats with myeloma-related disorders, cats with cutaneous plasmacytomas that underwent excision of the masses had a prolonged median survival of 663 days vs. 93 days in cats that received corticosteroids alone or no treatment. However, case reports have also suggested that some cutaneous EMPs may have a more aggressive behavior in cats.8


Most cases of SOP reported in dogs ultimately progress to multiple myeloma. However, these patients may have long disease-free intervals before disease progression.11,12 The optimal treatment for most patients with SOP is radiation therapy administered at a total dose of 40 to 50 Gy given in daily fractions for three to four weeks.11,14,26 Depending on the tumor's location, surgical excision may also be an option.12 Systemic chemotherapy can be considered at the time of local treatment of SOP but is controversial in the absence of systemic involvement.22


In general, EMPs often have a benign clinical course in dogs and cats and may be cured with complete surgical excision. Noncutaneous, non-oral EMP may have a more aggressive behavior; however, dogs treated with surgery alone or a combination of surgery and adjuvant systemic chemotherapy can have extended survival times. Most cases of SOP ultimately progress to multiple myeloma. Thorough staging before pursuing therapy for noncutaneous, non-oral EMP and SOP is important to rule out systemic disease.

Rachel Sternberg, DVM
Jackie Wypig, DVM DACVIM (oncology)
Department of Veterinary Clinical Medicine
College of Veterinary Medicine
University of Illinois
Urbana, IL 61802

Anne M. Barger, DVM, DACVP
Department of Pathology
College of Veterinary Medicine
University of Illinois
Urbana, IL 61802


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