Multiple Myeloma in a 9-year-old Labrador: Anatomic pathology perspective
Osteolytic lesions are characteristic of multiple myeloma and are seen in more than 50% of dogs with the IgG and IgA classifications.1,2 Grossly, multiple myeloma appears as dark red to brown, gelatinous masses replacing bone and marrow, usually in multiple locations. Commonly affected sites of osteolysis include the vertebrae, ribs, pelvis, skull and proximal metaphyses of long bones (Figures 11 and 12).3 Lysis and proliferation of the tumor within vertebral bodies can cause extradural compression of the spinal cord, as seen in the current case (Figure 13).4,5 Further dissemination can occur and affect the spleen, kidney, liver and lymph nodes (Figure 14).1
When evaluating a biopsy or tissue at necropsy, plasmacytosis in bone marrow is the diagnostic criterion used by anatomic pathologists. A section of bone marrow that is more than 30% plasma cells with concurrent osteolytic bone lesions, monoclonal gammopathy or Bence Jones proteinuria is highly suggestive of multiple myeloma.4 Changes in plasma cell morphology, such as binucleation, multinucleation and variation in cell or nuclear size (anisocytosis and anisokaryosis, respectively) are also suggestive of multiple myeloma.5 Immunohistochemistry may be used to confirm the identity of the neoplastic plasma cells and to better estimate the percent of plasma cells within bone marrow. Canine plasma cells are immunoreactive for multiple myeloma oncogene 1 (MUM1) with strong nuclear and weak cytoplasmic immunoreactivity.6 Immunohistochemistry may also be used to match either IgG or IgA to the clinically diagnosed monoclonal gammopathy.1
There are four described histologic patterns of neoplastic cells in canine bone marrow: focal, interstitial, mixed and packed.7 The interstitial pattern spares the fat and hematopoietic cells, while the packed pattern replaces fat and hematopoietic cells within the bone marrow. The mixed pattern is a combination of interstitial and packed. The packed pattern is associated with increased mitotic rate, as well as decreased survival time in dogs with multiple myeloma.7
In the current case, the histologic appearance of the bone marrow was consistent with a packed pattern, with sheets of neoplastic cells obliterating normal bone marrow elements (Figure 15). The neoplastic cells resembled normal plasma cells (round, eccentric nuclei, moderate eosinophilic cytoplasm and perinuclear clearing) and had mild anisocytosis and anisokaryosis, with seven mitotic figures per 10 high-power (400X) field (Figure 16).
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Matus RE, Leifer CE, MacEwen EG, et al. Prognostic factors for multiple myeloma in the dog. J Am Vet Med Assoc 1986;188(11):1288-1292.
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Muñoz A, Riber C, Satué K, et al. Multiple myeloma in horses, dogs and cats: a comparative review focused on clinical signs and pathogenesis. In: Hajek R, ed. Multiple myeloma—A quick reflection on the fast progress. 1st ed. InTech, 2013. Available at: https://www.intechopen.com/books/multiple-myeloma-a-quick-reflection-on-the-fast-progress/multiple-myeloma-in-horses-dogs-and-cats-a-comparative-review-focused-on-clinical-signs-and-pathogen.
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