Page 1319 - Small Animal Internal Medicine, 6th Edition

CHAPTER 78 Approach to the Patient With a Mass 1291

Clinicopathologic Features and Diagnosis
When evaluating a cat or dog with an AMM, the clinician
VetBooks.ir should consider several issues before recommending a spe-
cific treatment. As discussed previously (see Chapter 75), the
treatment recommended depends on the specific tumor type
(i.e., surgical excision may be curative for dogs and cats with
thymomas, whereas chemotherapy is indicated for those
with lymphoma). Because lymphomas and thymomas are
the most common AMMs in small animals, the ensuing
discussion is limited to these two neoplasms. Other neo-
plasms that originate in anterior mediastinal structures
include chemodectomas (heartbase tumors), ectopic thyroid
carcinomas, and lipomas, among others. Nonneoplastic
lesions of the mediastinum include mainly thymic or medi-
astinal hematomas, granulomatous lymphadenitis (e.g., blas-
tomycosis), and ultimobranchial cysts. FIG 78.2
Paraneoplastic syndromes such as generalized or focal Typical radiographic features of thymoma (arrows) in a
myasthenia gravis, polymyositis, exfoliative dermatitis, lym- dog. The mass originates in the ventral mediastinum, unlike
phocytosis, neutropenia, and second neoplasms have been most lymphomas, which usually originate in the dorsal
mediastinal region. Percutaneous fine-needle aspiration of
well characterized in cats and dogs with thymoma. Aplastic this mass yielded findings diagnostic for thymoma, and the
anemia, a paraneoplastic syndrome common in humans dog underwent a thoracotomy with complete resection of
with thymoma, is uncommon in small animals with this the mass.
tumor type. Hypercalcemia is a common finding in dogs
with mediastinal lymphoma, but it can also occur in dogs
with thymoma. the finding cannot be used as a means to distinguish between
In cats, the age at the time of presentation points to a these two tumor types. However, neoplastic cells are often
specific diagnosis. In other words, anterior mediastinal lym- seen in the pleural effusion in dogs and cats with lymphoma
phomas are more common in young cats (1-3 years old), but are typically absent in those with thymoma.
whereas thymomas are more common in older cats (>8 years Ultrasonographic evaluation of the AMM should be
old). It is also important to know the feline leukemia virus attempted before more invasive diagnostic techniques are
(FeLV) status in this species because most cats with medias- used. Ultrasonographically, most thymomas have mixed
tinal lymphomas are viremic (i.e., FeLV positive), whereas echogenicity, with discrete hypoechoic to anechoic areas that
most cats with thymoma are not. FeLV-negative mediastinal correspond to true cysts on cross section (Patterson and
lymphomas have been described in young to middle-age Marolf, 2014). The lack of a supporting stroma in lympho-
Siamese cats and can occasionally be seen in other cats. mas usually confers a hypoechoic to anechoic density to the
In dogs most AMMs are diagnosed in older animals mass, which therefore may look diffusely cystic. In addition
(older than 5-6 years of age); therefore age cannot be used to aiding in the presumptive diagnosis of a given tumor
as a means of distinguishing between lymphomas and thy- type, ultrasonography may provide information regard-
momas. However, a large proportion of dogs with mediasti- ing the resectability of the mass and assists in obtaining
nal lymphoma are hypercalcemic, whereas most dogs with a specimen for cytologic evaluation (see next paragraph).
thymomas are not (although hypercalcemia can also occur In patients with thymoma, a thoracic CT scan may help in
in dogs with this neoplasm). Peripheral blood lymphocytosis planning surgery.
can be present in dogs and cats with either lymphoma or Transthoracic FNA of AMMs constitutes a relatively safe
thymoma; in most dogs with lymphoma, the circulating lym- and reliable evaluation technique. After sterile preparation
phocytes are morphologically abnormal. The presence of of the thoracic wall overlying the mass (see Chapter 74),
neuromuscular signs in a dog or cat with an AMM suggests a 2- to 3-inch (5- to 7.5-cm), 25-gauge needle is used to
the existence of a thymoma or of lymphoma with central sample the mass. This can be done blindly (if the mass is
nervous system involvement. so large that it is pressing against the interior thoracic wall)
Thoracic radiographs are of little help in differentiating or guided by radiography (using three views to establish
thymomas from lymphomas. The two neoplasms are similar a three-dimensional location), fluoroscopy, ultrasonog-
in appearance, although lymphomas appear to originate raphy, or CT. Despite the fact that there are large vessels
more frequently in the dorsal anterior mediastinum, whereas within the anterior mediastinum, postaspiration bleeding
thymomas originate more often in the ventral mediastinum is extremely rare if the animal remains motionless during
(Fig. 78.2). Thymomas also occasionally “hug” the heart in the procedure. Alternatively, if the mass is large enough
the ventrodorsal radiographic view and can have sharp or to be in close contact with the internal thoracic wall, a
irregular edges. The prevalence of pleural effusion in dogs transthoracic needle biopsy can be performed to allow
and cats with either thymoma or lymphoma is similar; thus histopathologic evaluation.