Page 1320 - Small Animal Internal Medicine, 6th Edition

1292 PART XII Oncology

Cytologically, mediastinal lymphomas are composed of a respiratory compromise can further compound their clinical
monomorphic population of lymphoid cells that are mostly status.
VetBooks.ir immature (i.e., low nuclear-to-cytoplasmic ratio, dark blue usually curative. Although in some reports the perioperative
Because most thymomas are benign, surgical excision is
cytoplasm, clumped chromatin pattern, and nucleoli); in
cats, most cells in anterior mediastinal lymphomas are
et al., 1994), in the authors’ experience, most patients that
heavily vacuolated and resemble human Burkitt lymphoma morbidity and mortality of this procedure are high (Atwater
cells (Fig. 78.3). Thymomas are cytologically heterogeneous undergo thoracotomies for removal of a thymoma do well
and composed primarily of a population of small lympho- and are released from the hospital in 3 to 4 days. In a review
cytes (although large blasts are sometimes present) and occa- of the surgical outcome in 9 cats and 11 dogs with thymomas
sionally a distinct population of epithelial-like cells that are (Zitz et al., 2008), 8 out of 9 cats and 8 out of 11 dogs sur-
usually polygonal or spindle-shaped, which can be identified vived the immediate postoperative period and had median
either as individual cells or in sheets. Hassall corpuscles are survival times of 30 and 18.5 months, respectively. Two cats
rarely seen in Wright-stained cytologic preparations. Plasma and one dog had late recurrences.
cells, eosinophils, neutrophils, mast cells, macrophages, and Radiotherapy can successfully induce remission in
melanocytes are all occasionally seen (Fig. 78.4). Flow patients with thymoma, although complete, long-lasting
cytometry can also be used to help differentiate mediastinal remission is rarely achieved. This may be due to the fact
lymphoma from thymomas in dogs. The presence of >10% that radiotherapy eliminates only the lymphoid component
lymphocytes that are CD4+/CD8+ is characteristic for thy- of the neoplasm, whereas the epithelial component remains
momas, whereas mediastinal lymphomas generally con- unchanged. Chemotherapy may be beneficial in selected cats
tained <2% CD4+CD8+ lymphocytes (Lana et al., 2006). and dogs with nonresectable thymomas or in those in which
repeated anesthetic episodes or a major surgical procedure
Treatment poses a severe risk. One author’s clinic has used combina-
As discussed in preceding paragraphs, anterior mediastinal tion chemotherapy protocols commonly used for dogs and
lymphomas are best treated with chemotherapy (see Chapter cats with lymphoma (i.e., cyclophosphamide, vincristine,
79). Radiotherapy can also be used in conjunction with cytosine arabinoside, and prednisone [COAP]; cyclophos-
chemotherapy to induce a more rapid remission. A recent phamide, vincristine, and prednisone [COP]; and cyclophos-
publication on canine mediastinal lymphoma reported that phamide, doxorubicin, vincristine, and prednisone [CHOP];
two dogs that did undergo radiotherapy in addition to their see Chapter 79) in a limited number of cats and dogs with
chemotherapy protocol had prolonged progression-free sur- cytologically diagnosed thymomas. As with radiotherapy,
vival and overall survival compared with the rest of the dogs however, chemotherapy may only eliminate the lymphoid
in the study that did not undergo radiation (Moore et al., cell population, thus rarely resulting in complete or long-
2018). However, the animal must be deemed an appropriate lasting remissions.
candidate for anesthesia before consideration of radiother- If a definitive diagnosis of thymoma or lymphoma cannot
apy, because chemical restraint in patients exhibiting severe be obtained preoperatively, the clinician has two therapeutic

FIG 78.4
FIG 78.3 Cytologic characteristics of canine thymoma. Note the
Cytologic characteristics of feline mediastinal lymphoma. heterogeneous lymphoid cell population, which also
Note the dark cytoplasm with abundant vacuoles typical of includes neutrophils and mast cells (×1000). (Courtesy Dr.
this neoplasm in cats (×1000). D. Pappas.)