Page 757 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 33 Hematopoietic Tumors 735

in all bone marrow cells, signifying a lesion at the level of an PV, should be ruled out, and importantly, there should be no
early multipotent stem cell. Typically, these individuals have primary disorders associated with reactive thrombocytosis, 710
including inflammation, hemolytic anemia, iron deficiency ane-
a chromosomal translocation, resulting in the Philadelphia
VetBooks.ir chromosome or BCR–ABL translocation between chromo- mia, malignancies, recovery from severe hemorrhage, rebound
699
The analogous chromosomes in dogs are
somes 9 and 22.
from immune-mediated thrombocytopenia, and splenectomy.
chromosomes 9 and 26, and BCR–ABL translocation, termed In addition, certain drugs such as vincristine can induce throm-
the “Raleigh chromosome,” has been reported in several cases bocytosis. ET has been recognized in dogs. 642,711–714 In one dog,
of CML in dogs. 614,627,700,701 Variants of CML are chronic the platelet count exceeded 4 million/μL and bizarre giant forms
myelomonocytic leukemia and chronic monocytic leukemia with abnormal granulation were present. The bone marrow con-
(CMoL). 702–704 CMoL has also been associated with BCR–ABL tained increased numbers of megakaryocytes and megakaryo-
translocation in the dog. 702 These diagnoses are made based on blasts, but circulating blast cells were not seen. Other findings
the percentage of monocytes in the leukemic cell population. included splenomegaly, GI bleeding, and increased numbers of
An infrequent myeloproliferative neoplasm, atypical chronic circulating basophils. Causes of secondary or reactive throm-
myeloid leukemia, has been reported in a dog and had features bocytosis were ruled out. 713 Basophilia was also reported in a
of both myelodysplastic syndrome and chronic leukemia. 700 In more recent case. 711 In another dog, ET was diagnosed and then
this dog, BCR–ABL translocation was present in fewer than progressed to CML. 642 In some cases reported in the literature
10% of cells, considered a negative finding. as ET, the dogs had microcytic hypochromic anemias. Because
In addition to accumulating in bone marrow and periph- iron deficiency anemia is associated with reactive or secondary
eral blood, leukemic cells also are found in the red pulp of thrombocytosis, care must be taken to rule out this disorder.
the spleen, the periportal and sinusoidal areas of the liver, However, spurious microcytosis may be reported if a dog has
and sometimes lymph nodes. Other organs, such as the kid- many giant platelets that are counted by some analyzers as small
ney, heart, and lung, are less commonly affected. In addition, RBCs. 712 Microscopic review of the blood film may be helpful
extramedullary hematopoiesis may be present in the liver and in these cases.
spleen. Death is usually due to complications of infection or
hemorrhage secondary to neutrophil dysfunction and throm-
bocytopenia, respectively. In some cases, CML may terminate Other Bone Marrow Disorders
in “blast crisis,” in which there is a transformation from a Myelofibrosis
predominance of well-differentiated granulocytes to excessive
numbers of poorly differentiated blast cells in peripheral blood Primary myelofibrosis has been reported only rarely in dogs,
and bone marrow. This phenomenon is well documented in the and myelofibrosis is more typically a secondary, or reactive, pro-
dog. 693,696,698 cess. 715,716 In humans, myelofibrosis is characterized by collagen
deposition in bone marrow and increased numbers of megakaryo-
Basophilic and Eosinophilic Leukemia cytes and granulocytic precursors, many of which exhibit mor-
phologic abnormalities. In fact, breakdown of intramedullary
Basophilic leukemia, although rare, has been reported in dogs and megakaryocytes and subsequent release of factors that promote
is characterized by an increased WBC count with a high propor- fibroblast proliferation or inhibit collagen breakdown may be the
tion of basophils in peripheral blood and bone marrow. 705–707 underlying pathogenesis of the fibrosis. 717 Focal osteosclerosis is
Hepatosplenomegaly, lymphadenopathy, and thrombocytosis sometimes present. Anemia, thrombocytopenia, splenomegaly,
may be present. All the dogs have been anemic. Basophilic leu- and myeloid metaplasia (production of hematopoietic cells out-
kemia should be distinguished from mast cell leukemia (masto- side the bone marrow) are consistent features.
cytosis). Whether dogs develop eosinophilic leukemia remains in In dogs, myelofibrosis occurs secondary to MPDs, radia-
question. Reported cases have had high blood eosinophil counts tion damage, and congenital hemolytic anemias. 718–721 In
and eosinophilic infiltrates in organs. 708,709 One dog responded some cases, the inciting cause is unknown (idiopathic myelo-
well to treatment with corticosteroids. The distinction between fibrosis). There may be concurrent marrow necrosis in cases of
neoplastic proliferation of eosinophils and idiopathic hypereosin- ehrlichiosis, septicemia, or drug toxicity (estrogens, cephalo-
ophilic syndrome remains elusive. Nonmyeloproliferative disor- sporins), and there is speculation that fibroblasts proliferate in
ders associated with eosinophilia such as parasitism, skin diseases, response to release of inflammatory mediators associated with
or diseases of the respiratory and GI tracts should be considered the necrosis. 715 Myeloid metaplasia has been reported to occur
first in an animal with eosinophilia. One distinguishing feature in the liver, spleen, and lung. 721 Extramedullary hematopoiesis
should be clonality, with reactive eosinophilia comprising poly- is ineffective in preventing or correcting the pancytopenia that
clonal cells and the neoplastic condition arising from a single eventually develops.
clone. As clonality assays become more available, this discrepancy
may be resolved. Myelodysplastic Syndrome

Essential Thrombocythemia Dysfunction of the hematopoietic system can be manifested by a
variety of abnormalities that constitute myelodysplastic syndrome
In humans, ET, or primary thrombocytosis, is characterized by (MDS). In dogs, in which the syndrome is rare, there usually are
platelet counts that are persistently greater than 600,000/μL. cytopenias in two or three lines in the peripheral blood (anemia,
There are no blast cells in circulation and marked megakaryocytic neutropenia, and/or thrombocytopenia). Other blood abnormali-
hyperplasia of the bone marrow without myelofibrosis is pres- ties can include macrocytic erythrocytes and metarubricytosis.
ent. Thrombosis and bleeding are the most common sequelae, The bone marrow is typically normocellular or hypercellular, and
and most patients have splenomegaly. Other MPDs, especially dysplastic changes are evident in several cell lines. If blast cells are