Page 752 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

730 PART IV Specific Malignancies in the Small Animal Patient

reveal characteristic ultrastructural features. The French–Ameri- • BOX 33.6 Key Clinical Summary Points: Myeloid

can–British (FAB) classification system is considered useful in Leukemia, Myeloproliferative
cats with myelodysplastic syndromes and almost all reported
VetBooks.ir cases have been FeLV antigenemic. 607,608 • Myeloid leukemias and myeloproliferative disorders are rare neoplastic
Neoplasms, and Myelodysplasia

Lymphoid Leukemia diseases of bone marrow in which there are clonal disorders of
hematopoietic stem cells (Table 33.13).
ALL was the most commonly encountered type of leukemia in • Clonal disorders of bone marrow include myeloaplasia (usually referred
cats in the FeLV era; however, it is much less common today. ALL to as aplastic anemia), myelodysplasia, and myeloproliferation.
is characterized by poorly differentiated lymphoblasts and pro- • The terms acute and chronic refer to the degree of cellular
lymphocytes in blood and bone marrow. Approximately 60% to differentiation of the leukemic cells, but these terms also correlate with
80% of cats with ALL are FeLV positive, and most malignant cells the biologic behavior of the neoplasm.
have T-cell immunophenotypes 609 ; however, little information is • The acute myeloid leukemias (AMLs) are aggressive leukemias that
progress rapidly, are poorly responsive to treatment, and are associated
available in the contemporary literature. with short survival times (typically <2 months).
CLL is rarely reported in cats and is characterized by well-dif- • In AML, determination of the leukemic lineage (i.e., neutrophilic,
ferentiated, small, mature lymphocytes in peripheral blood and monocytic, erythroid, or megakaryocytic origin) requires advanced
bone marrow. 606,607,610,611 In the largest compilation, most cats diagnostic testing (cytochemical staining, flow cytometric analysis,
were older, half were presented incidentally, and half with nonspe- clonality testing, or genetic analysis).
cific constitutional signs (weight loss, hyporexia). 606 No specific • Myeloproliferative neoplasms (MPNs), previously termed chronic
serum biochemical abnormalities were noted; however, 50% were myeloproliferative disorders, are characterized by excessive production
anemic, 10% thrombocytopenic, and all had peripheral lympho- of differentiated bone marrow cells, resulting in the accumulation
cyte counts of greater than 9000/μL (although this was the cutoff of erythrocytes (polycythemia vera), granulocytes and/or monocytes
required for diagnosis). All cats tested (n = 13) were negative for (chronic myelogenous leukemia and its variants), or platelets (essential
thrombocythemia).
FeLV/FIV. Median peripheral lymphocyte counts were 34,200/μL • Because of the degree of differentiation of cells in MPNs, these
(range, 9 to >300,000/μL). Whereas most are of the T-cell lin- disorders must be distinguished from nonneoplastic causes of increases
eage, B-cell CLL has also been reported in cats. 606,607,610,611 In one in these cell types.
report of 18 cases of CLL in cats, most were found to be T-helper • Compared with AMLs, MPNs and myelodysplasias have a more chronic
cells (CD3 /CD4 /CD8 ). course and may have better responses to therapy, although most will
+
+
– 611
progress and ultimately result in mortality.
Treatment of Leukemias
The use of chemotherapy to treat ALL has been disappointing.
Using COP-based protocols, a 27% CR rate has been reported. 612 monocytic, erythroid, megakaryocytic, or mixed). Newer classifi-
CLL can be treated with chlorambucil (0.2 mg/kg PO or 2 mg/ cation systems in humans have incorporated genetics and molecu-
cat qod; alternatively, 20 mg/m q2 weeks) and prednisolone (1 lar genetic analysis; these are currently areas of active investigation
2
mg/kg PO daily). In 16 cats treated with chlorambucil and pred- in the study of animal MPDs. 614 In 1991, the Animal Leukemia
nisolone, approximately 90% responded with a median remission Study Group made recommendations for classifying nonlym-
duration of 6 months; however, half achieved CR with a median phoid leukemias in dogs and cats. 615 More recently, the Oncology
remission duration of 14 months. 606 Several of these cats were Committee of the American College of Veterinary Pathologists
rescued with various protocols after recurrence, many with pro- (ACVP) has been reexamining criteria for a classification system
longed second remissions. As in humans and dogs, if significant and spearheading large multiinstitutional studies to validate the
clinical signs or profound cytopenias are not present, treatment criteria. Long-term objectives of these studies are to define molec-
can be withheld; one cat with CLL remained stable without che- ular lesions, establish prognostic markers, and identify effective
motherapy for over a year. 610 therapeutic approaches. 616 See Box 33.6.
The prognoses for feline ALLs are generally very poor, although
some exceptions exist in case report form in the historic literature. Incidence and Risk Factors

Myeloid neoplasms are uncommon or rare in the dog and occur
SECTION C: CANINE ACUTE MYELOID 10 times less frequently than lymphoproliferative disorders. 617
LEUKEMIA, MYELOPROLIFERATIVE Accurate information about incidence and other epidemiologic
NEOPLASMS, AND MYELODYSPLASIA information has been generally lacking owing to a lack of a consis-
tent use of a uniform classification system; however, several larger
compilations have been published recently. 618–622 There is no con-
DAVID M. VAIL AND KAREN M. YOUNG sistent breed predilection; most are large-breed dogs with a median
age of 7 to 8 years, although acute myeloid leukemia (AML) can
Myeloproliferative disorders (MPDs) are a group of neoplastic occur in dogs as young as 7 months of age. AML occurred more
diseases of bone marrow in which there are clonal disorders of frequently in males than females (2:1 ratio) in two large compi-
hematopoietic stem cells. 613 Aberrant proliferation of cells with lations. 618,621 In dogs, the etiology of spontaneously occurring
defective maturation and function leads to reduction of normal leukemia is unknown. It is likely that genetic and environmental
hematopoiesis and invasion of other tissues. These disorders have factors (including exposure to radiation, drugs, or toxic chemicals)
been classified based on biologic behavior, degree of cellular dif- play a role. In humans, acquired chromosomal derangements lead
ferentiation, and lineage of the neoplastic cells (granulocytic, to clonal overgrowth with arrested development. 623 Chromosomal