Page 215 - BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery, 2nd Edition

BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery

As previously mentioned, paraneoplastic syndromes lymph nodes, diaphragm and pericardium. Distant meta-
have been observed in association with thymic neoplasia stasis is rare but has been reported to the kidneys, liver
VetBooks.ir thenia gravis and megaoesophagus and less commonly seen occasionally (Figure 16.9).
and spleen. Local invasion of the vascular system is also
in humans, dogs and cats, most notably acquired myas-
The prognosis is good for animals with non-invasive
hypercalcaemia and other immune-mediated diseases
(Bellah et al., 1983; Atwater et al., 1994). A large percent-
age of human patients with myasthenia gravis have non-metastatic thymoma following complete excision (pro-
viding that paraneoplastic syndromes are not present),
thymic abnormalities, either neoplastic or non-neoplastic whereas the prognosis for animals with invasive or meta-
(Robertson et al., 1998; Mantegazza et al., 2003). The inci- static thymoma is poor. Theoretically, preoperative assess-
dence of acquired myasthenia gravis in dogs with thy- ment of local disease should be critical to clinical
moma is approximately 40% (Bellah et al., 1983; Aronsohn, decision-making. The degree of local tissue invasion is dif-
1985; Atwater et al., 1994). ficult to determine on the basis of plain thoracic radio-
graphy, so advanced imaging such as MRI and CT is often
recommended to provide additional information about the
Malignant mediastinal lymphadenopathy local disease. Unfortunately, many animals are presented
Mediastinal lymphadenopathy affecting the sternal, cranial with extremely large mediastinal masses, which compli-
mediastinal or tracheobronchial lymphocentra is most cates differentiation between invasion and ‘mass effect’
commonly due either to multicentric neoplasia such as (Fujimoto et al., 1992; Pirronti et al., 2002) (Figure 16.10);
lymphoma, mastocytosis or malignant histiocytosis or to consequently, it may be impossible to differentiate invasive
metastatic disease. All of the neoplastic causes are con- from non-invasive forms of thymoma on the basis of diag-
sidered multicentric and, as such, surgical excision is of no nostic imaging alone.
therapeutic value once a diagnosis is confirmed. With the
Tumour embolism within
exception of lymphoma, they are poorly responsive to anti- Tumour embolism within
cranial vena cava
cancer drug protocols (see the BSAVA Manual of Canine cranial vena cava
and Feline Oncology).
Thymic neoplasia
Thymoma and thymic carcinoma are the most commonly
reported primary thymic tumours in dogs and cats. Of
these, thymoma is the most common.

Clinical features
The clinical features of thymoma are vague and often mild
until compression of the lungs and/or airways causes
breathing difficulty. Occasionally, animals with this disease
will present with signs referable to a paraneoplastic syn-
drome such as myasthenia gravis or hypercalcaemia.
Intraoperative photograph of the case in Figure 16.8. A large
16.9 neoplastic embolism can be seen within the cranial vena cava.
Diagnosis
The diagnosis is often suspected on the basis of plain
thoracic radiographs and may be confirmed by cytological Vessels
examination of fine-needle aspirates. Cytological evalua-
tion of fine-needle aspirate samples has been shown to
correlate well with the definitive histological diagnosis
(Reichle and Wisner, 2004; Pintore et al., 2014). Thymoma
and lymphoma may both contain large numbers of
lymphocytes, making definitive diagnosis by examination
of fine-needle aspirates alone challenging. Core biopsies
or excisional biopsies are occasionally required to achieve
a definitive diagnosis (Zitz et al., 2008).

Biological behaviour

The biological behaviour of thymoma does not correlate
well with the histological appearance of the neoplasm.
Several histological subtypes of thymoma are described,
epithelial, lymphocyte-rich and clear cell, among which
lymphocyte-rich thymoma may have a better prognosis
(Atwater et al., 1994; Zitz et al., 2008). Because the histo- Thymoma Trachea
logical appearance of thymoma does not correlate well
with the biological behaviour, thymomas are predominantly
classified on the presence or absence of local invasion. 16.10 T2-weighted magnetic resonance image through the cranial
thorax of a cat, cranial to the heart. The massive volume of the
Thymomas are, therefore, described as ‘non-invasive’ or soft tissue mass makes differentiation of mass effect from invasiveness
‘invasive’, on the basis of gross local tissue invasion. Local di cult. ote that blood vessels appear black because moving fluid is
metastases are uncommon but can be seen in the lungs, black on T images compared ith free fluid hich is hite.

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