Page 762 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

740 PART IV Specific Malignancies in the Small Animal Patient

• BOX 33.7 Key Clinical Summary Points: 14% of abnormal and malignant serum protein electrophoretic
768
Myeloma-Related Disorders samples, respectively, in a compilation of 155 feline samples.
MM occurs in aged cats (median age 12–14 years), most com-
VetBooks.ir • Myeloma-related disorders (MRDs) represent clonal neoplastic monly in domestic short hair cats, and no sex predilection has
been consistently reported, although a male preponderance may
populations of plasma cells.
• They include multiple myeloma (MM), extramedullary plasmacytoma exist. 764,767,769–771 MM has not been associated with coronavirus,
(EMP), (Waldenström’s) macroglobulinemia, solitary osseous FeLV, or FIV infections.
plasmacytoma (SOP), and plasma cell leukemia. The etiology of MM is for the most part unknown. Genetic
• Dogs and cats with MM are presented with a wide variety of clinical predispositions, molecular aberrations (e.g., c-kit), viral infections,
signs (Tables 33.14, 33.15) and clinicopathologic abnormalities (Table chronic immune stimulation, and carcinogen exposure have all been
33.16) resulting from high levels of circulating M component, organ or suggested as contributing factors. 764,772–779 Suggestion of a familial
bone infiltration with neoplastic cells, or both.
• Diagnosis of MM usually follows the demonstration of bone marrow or association in cats follows cases reported among siblings. 770 Evi-
visceral organ plasmacytosis (Fig. 33.23), the presence of osteolytic dence exists that molecular mechanisms of cellular control, includ-
bone lesions (Figs. 33.25, 33.26), and the demonstration of serum or ing overexpression of cell cycle control components like cyclin D1
urine myeloma proteins (M component; Fig. 33.24). (see Chapter 2), and receptor tyrosine kinase dysregulation may be
• Most (>80%) dogs with MM respond to chemotherapy (melphalan/ involved in canine MM and plasma cell tumors. 774,776 In rodent
prednisone) and enjoy durable remissions, with median survival times of models, chronic immune stimulation and exposure to implanted
1.5 to 2.5 years. Most (50%–80%) cats also respond to chemotherapy silicone gel have been associated with development of MM, 778,779
(cyclophosphamide/prednisolone) although the durability of response tends as have chronic infections and prolonged hyposensitization therapy
to be shorter, with median survival times reported from 4 to 13 months in humans. 775 Viral Aleutian disease of mink results in monoclo-
• Cutaneous and oral solitary EMPs are usually cured after surgical nal gammopathies in a small percentage of cases. 777 Exposure to
excision.
• Cutaneous plasmacytosis, however, is associated with multiple lesions the agricultural industry, petroleum products, and irradiation are
(10s to 100s), is a biologically aggressive disease, with treatment and known risk factors for development in humans. 780–782 In addition,
outcomes more like those for MM. progression of solitary plasma cell tumors to MM has been reported
• Noncutaneous/nonoral EMPs and SOPs can be initially confined in both dogs and cats, and a single case of a B-cell lymphoma pro-
to local sites and respond to local therapy (i.e., surgery, radiation gressing to MM exists in the dog. 783–785
therapy); however, frequent rechecks are necessary, because many will
eventually progress systemically.
Pathology and Natural Behavior
MM is a systemic proliferation of malignant plasma cells or their
precursors arising as a clone of a single cell that usually involves
(Waldenström’s) macroglobulinemia, solitary osseous plasmacy- multiple bone marrow sites in dogs. In cats, as previously stated,
toma (SOP), and Ig-secreting lymphomas and leukemias (includ- a blurring of the distinction of MM and multicentric noncutane-
ing plasma cell leukemia). MM is the most important MRD based ous EMP within the MRD occurs because widespread abdominal
on clinical incidence and severity. There appears to be some dis- organ involvement without significant bone marrow infiltration
cordance and blurring of the distinction between MM and mul- has been described in a proportion of cases in European com-
ticentric noncutaneous EMP in cats and these two MRDs will be pilations. 771,786 Because both MM and multicentric noncutane-
discussed together in this species. See Box 33.7. ous EMP have a similar clinical course and widespread systemic
involvement with hyperglobulinemia in cats, they will be dis-
Multiple Myeloma cussed as MM in this chapter. Malignant plasma cells can have
a varied appearance on histologic sections and cytologic prepara-
Incidence and Etiology tions. The degree of differentiation ranges from those resembling
normal plasma cells in late stages of differentiation (Fig. 33.23) to
Although MM represents fewer than 1% of all malignant tumors very large anaplastic round cells (often referred to as plasmablasts)
in animals, it is responsible for approximately 8% of all hemato- with a high mitotic index representing early stages of differen-
poietic tumors and 3.6% of all primary and secondary tumors tiation. 763,764,767,786 Binucleate and multinucleate cells are often
affecting bone in dogs. 757,758 In a compilation of bone marrow present (see Fig. 7.32, Chapter 7). In 16 cats with MM in one
disorders in dogs (n = 717), MM represented 4.4% and 19.8% case series, 787 the majority (83%) of plasma cells were immature
of all abnormal samples and neoplastic processes, respectively. 759 and had marked atypia, including increased size, multiple nuclei,
Furthermore, in a compilation of serum protein electrophoretic clefted nuclei, anisocytosis, anisokaryosis, variable nuclear : cyto-
samples (n = 147 dogs), MM accounted for 4.3% of abnormal plasmic ratios, decreased chromatin density, and variable nucleoli;
and 28.5% of neoplastic processes encountered, respectively. 760 nearly one quarter had “flame cell” morphology characterized by
Several compilations have suggested a male predisposition, 761–763 peripheral eosinophilic cytoplasmic processes. 767 However, in a
whereas others have not observed this. 758,764 Older dogs are European compilation of feline multicentric noncutaneous MRD
affected with an average age of between 9 and 10 years (range, cases (n = 17), 78% had well-differentiated morphologies. 786 The
3–14 years). 758,761–764 In one large case series, German shepherd authors of this latter case series developed a grading system depen-
dogs were overrepresented based on the hospital population. 758 dent on the percentage of plasmablasts within the neoplastic cells
The true incidence of MM in the cat is unknown; however, it is in which well-differentiated, intermediate-grade, and poorly dif-
a more rare diagnosis than in the dog, representing only 1 of 395 ferentiated MMs have less than 15%, 15% to 49%, and 50% or
and 4 of 3248 tumors in two large compilations of feline malig- more plasmablasts, respectively. 786 Malignant plasma cells typi-
nancies, and 0.9% of all malignancies and 1.9% of hematologic cally produce an overabundance of a single type of or compo-
malignancies in another report. 765–767 MM represented 1.4% and nent of immunoglobulin, which is referred to as the M component