Page 807 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 34 Miscellaneous Tumors 785

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A B
• Fig. 34.8 (A) Pleural, parietal, and pericardial surfaces of a dog at necropsy illustrating nodular lesions
histologically confirmed as mesothelioma. (B) Mesothelioma involving the peritoneal surfaces of a cat at
necropsy.

greater risk has been related to amphibole asbestos compared malignant because of their ability to seed the body cavity, but dis-
with chrysotile exposure. 273 Chrysotile now accounts for 90% of tant metastasis is rare.
asbestos used worldwide. 273 Mesothelial cells appear morphologically as epithelial cells;
The underlying mechanisms of the neoplastic transformation however, their derivation is from mesoderm. Mesothelioma can
of mesothelial cells relates to the direct and indirect interaction appear histologically as epithelial, mesenchymal, or biphasic,
of asbestos with mesothelial cells and the chronic inflammatory which is a combination of the two. 282 The epithelial form, which
response initiated by activated macrophages attempting to engulf resembles carcinoma or adenocarcinoma, is by far the most com-
the asbestos fibers. The chronic inflammatory response also creates mon form in small animals. There are also several reports of a vari-
a distinct immunosuppressive tumor microenvironment, leading ation of the mesenchymal form, which resembles sarcoma and is
to infiltration with myeloid-derived suppressor cells, tumor-associ- referred to as sclerosing mesothelioma. 261,283–285 The biphasic form
ated macrophages, and regulatory T cells, which likely contributes of mesothelioma has been reported in two dogs and a cat. 286–288 A
to tumor progression. 274 Although a wide array of mutations have cystic peritoneal mesothelioma has also been reported in a dog. 289
been identified in mesotheliomas, the most frequent mutational This is a rare, benign, slowly progressive form of mesothelioma in
events involve inactivation of tumor suppressor genes, includ- humans, which is treated with surgical excision when the disease
ing cyclin-dependent kinase inhibitor 2A (CDKN2A), BRCA1 is localized.
association protein 1 (BAP1), and neurofibromin 2 (NF2). 275,276
Numerous cytokines including platelet-derived growth factor and History and Clinical Signs
vascular endothelial growth factor (VEGF) produced by stimu-
lated macrophages or mesothelial cells are likely important in the Mesotheliomas occur as a diffuse nodular mass or multifocal
pathogenesis of mesothelioma. 277 A report of five golden retriev- masses covering the surfaces of the body cavity (Fig. 34.8). Exten-
ers that developed pericardial mesothelioma after a long (30–54 sive effusions occur as a result of exudation from the tumor surface
months) history of idiopathic hemorrhagic pericardial effusion or from tumor-obstructed lymphatics; therefore the most com-
supports the concept that chronic inflammation may lead to neo- mon presenting sign is dyspnea from pleural effusion or a dis-
plastic transformation in canine mesothelial cells. 278 tended abdomen from peritoneal effusion. Dogs with pericardial
Mesothelial tumors occur most often in older animals; however, or heart-base mesotheliomas can present with tamponade and
in cattle and sheep, newborn or young animals may be affected. right-sided heart failure. 290–292
Juvenile mesothelioma has been reported in two mixed-breed dogs Sclerosing mesothelioma is a variation of mesothelial tumor
under 1 year of age; no underlying etiology was identified. 279,280 A seen primarily in male dogs, with German shepherd dogs over-
report of a 7-week-old puppy with mesothelioma suggests a con- represented. 261,283,284,293 These tumors present as thick fibrous lin-
genital form may exist. 281 ings in the abdominal and/or pleural cavities. Restriction occurs
around organs in the affected area, and in the abdomen such
Pathology and Natural Behavior changes can impinge on organs and lead to vomiting and urinary
tract signs.
The normal mesothelium is a monolayer of flattened mesothelial
cells. These cells are distinguished by the presence of microvilli, Diagnostic Techniques and Workup
desmosomes, and phagocytic potential. Disease conditions associ-
ated with inflammation or irritation of the lining of body cavities Mesothelioma should be suspected in adult dogs presenting with
commonly result in a marked proliferation of the mesothelial cells. a history of chronic, nonspecific clinical signs and fluid accumula-
Fluid accumulation in a body cavity promotes exfoliation and tion in any body cavity. Routine echocardiography and abdominal
implantation of mesothelial cells. Mesotheliomas are considered ultrasound are not typically helpful because the tumor cells cling
to epithelial surfaces and a mass lesion is uncommonly noted. 294