Page 428 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

406 PART IV Specific Malignancies in the Small Animal Patient

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dogs with FSAs were significantly younger than dogs with other fibroblasts. PWTs are characterized by the amounts and types
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histologic subtypes of STSs in another study. FSAs are more of cytoplasmic contractile proteins, which progressively increase
from pericytes in capillaries to myopericytes and smooth muscle
likely to recur after incomplete histologic excision and have higher
VetBooks.ir mitotic rates than other histologic subtypes of STSs 32–35 but, con- myocytes in the vascular subendothelial lining of larger vessels.
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PWTs have a characteristic cytologic appearance with moderate to
versely, are more likely to be low grade.
Pleomorphic Sarcoma (or Malignant Fibrous Histiocytoma) high cellularity, cohesion of spindle cells, presence of capillaries,
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and multinucleate cells. They are diagnosed histologically based
Malignant fibrous histiocytoma (MFH) is a tumor with histologic on vascular growth patterns (e.g., staghorn, placentoid, perivas-
characteristics resembling histiocytes and fibroblasts. Accord- cular whirling, bundles of media) and are further characterized
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ing to the World Health Organization classification of soft tissue by IHC staining patterns. Pericytes express vimentin and vari-
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tumors, the preferred term for MFH is undifferentiated pleo- able amounts of pan and α-smooth muscle actin; myopericytes are
morphic sarcoma because electron microscopic and IHC analy- characterized by the additional expression of desmin and calponin;
ses of these tumors have shown that the term “fibrohistiocytic” is and smooth muscle cells express smoothelin and heavy caldes-
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a misnomer. Pleomorphic sarcomas are typically diagnosed in mon. Based on IHC staining, canine PWTs have been classified
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middle-aged to older dogs. There is no sex predilection, although similarly to human PWTs, with the following being recognized:
in one report 70% of dogs with the giant cell variant of pleo- myopericytoma, angioleiomyoma, angioleiomyosarcoma, heman-
morphic sarcoma were female. Flat-coated retrievers, Rottwei- giopericytoma, angiofibroma, and adventitial tumor. 45,46 PWTs
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lers, and golden retrievers are overrepresented. 38,39 Pleomorphic are differentiated from PNSTs by both histology and IHC. 45,47
sarcomas is most commonly diagnosed in the subcutaneous tis- PWTs are characterized by a less aggressive biologic behavior, with
sues of the trunk and pelvic limbs and the spleen in dogs. Com- significantly lower rates of local recurrence than other histologic
puted tomography (CT) and magnetic resonance imaging (MRI) subtypes.
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characteristics of pleomorphic sarcomas have been described in
humans but not in dogs. In people, pleomorphic sarcomas are Tumors of Peripheral Nerves
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typically characterized as a large lobulated inhomogeneous hypo-
to isodense mass with inhomogeneous enhancement on CT Peripheral Nerve Sheath Tumor
and hypo- to isointense on T1-weighted images, with inhomo- PNSTs are tumors of nerve sheath origin, arising from Schwann
geneous enhancement, hyperintensity, and hypointense areas on cells, perineural cells, or perineural or endoneural fibroblasts.
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T2-weighted images on MRI. Four histologic subtypes of pleo- Benign and malignant variants have been described. The most
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morphic sarcoma are described: storiform–pleomorphic, myxoid, common benign PNSTs are schwannomas and neurofibromas,
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giant cell, and inflammatory. Definitive IHC staining patterns and these tend to be well circumscribed, located in the skin and
have not been established, but pleomorphic sarcomas will typically subcutaneous tissue, with an equal distribution of Antoni A and
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be vimentin positive and CD18 negative. Histologic subtype B histologic patterns. Malignant PNSTs are often subcutaneous,
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has prognostic significance in people, with the giant cell subtype poorly circumscribed, and invasive into deeper tissues, and associ-
having a higher local recurrence rate than storiform–pleomor- ated with high local tumor recurrence rates and relatively poor
phic subtype and a higher metastatic rate than the inflammatory survival times. 47,48 Malignant PNSTs can be differentiated from
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subtype. Giant cell pleomorphic sarcomas have been described PWTs based on IHC staining; malignant PNSTs stain positive
in 10 dogs: they were highly metastatic to subcutaneous tissue, with S-100, vimentin, glial fibrillary acidic protein (GFAP), nerve
lymph nodes (LNs), liver, and lungs; and the median survival time growth factor receptor, and neuron-specific enolase. 47–49 In addi-
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(MST) in these dogs was only 61 days. Canine pleomorphic tional, PNSTs have significantly higher Ki67 index than PWTs. 49
sarcomas are significantly more likely to be high grade and have Regardless of nomenclature, these tumors can occur anywhere
metastases at the time of diagnosis compared with other histologic in the body. Despite appearing encapsulated at surgery, they are
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subtypes of STSs. similar to FSAs and are usually poorly defined without histologic
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encapsulation. Most are adherent to deeper tissues and may infil-
Myxosarcoma trate underlying fascia, muscle, and skin. Although PNSTs are
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Myxosarcomas are neoplasms of fibroblast origin with an abun- considered malignant, they have a modest metastatic rate. Local
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dant myxoid matrix composed of mucopolysaccharides. These recurrence is common after conservative surgery. PNSTs tend to
rare tumors occur in middle-aged or older dogs and cats. The grow slowly and can range in size from 0.5 cm to greater than 12
majority are subcutaneous tumors of the trunk or limbs, but cm in diameter. In some cases, they can easily be confused with
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there are reports of myxosarcomas arising from the heart, eye, and lipomas on initial clinical examination. 29
brain. 42–44 These tumors tend to be infiltrative growths with ill- PNSTs of macroscopic nerves, which are not considered part
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defined margins. of the conventional classification of STSs in dogs, are classified
as peripheral, root, or plexus. Peripheral PNSTs involve mac-
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Tumors of the Vascular Wall roscopic nerves distant to either the brain or spinal cord, and
this form is much more amenable to treatment than either the
Perivascular Wall Tumor root or plexus PNSTs. Plexus PNSTs can involve either the bra-
PWTs are derived from the different cellular components of the chial or lumbrosacral plexus. The vast majority of cases will
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vascular wall, excluding the endothelial lining. The components show signs of unilateral lameness, muscle atrophy, paralysis, and
of the vascular wall depend on the type of vessel. Capillaries are pain. They can invade the spinal cord, especially high-grade
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composed of endothelium, pericytes, and basement membrane; root and plexus PNSTs. Treatment options include surgery,
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large veins and arteries are composed of endothelium, subendo- surgery with adjuvant RT, or RT alone. Surgical excision typi-
thelial lining cells, basement membrane, a medial layer of smooth cally involves forequarter amputation, although limb-sparing
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muscle cells, and an adventitial layer of myofibroblasts and nerve-specific compartmental resection is occasionally possible.