Page 216 - BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery, 2nd Edition

Chapter 16 · Surgery of the mediastinum

Treatment Recurrence rates are not well documented, but based
on the limited literature available, recurrence appears to
Surgical excision is indicated in the management of thy- occur in approximately 30% of dogs (Aronsohn 1985;
VetBooks.ir to confirm or refute the suspicion of local invasion based Gores et al., 1994). Cats seem to respond extremely well
moma, and exploration of the thorax is generally required
to surgical excision, with long-term remission. In a retro-
on imaging. Although exploration can be performed via
lateral thoracotomy, sternotomy is often preferable because
6–36-month follow-up period (Gores et al., 1994). A more
of the size of the tumour. Non-invasive thymomas can be spective study of 10 cats, no recurrence was observed in a
readily removed with blunt dissection (Figures 16.11 and recent study of thymoma in both dogs and cats docu-
mented a median survival time of 1825 days, with 1-year
16.12). Preservation of structures such as the phrenic
nerves and accurate haemostasis of larger vessels are and 3-year survival rates being 89% and 74% in cats (Zitz
et al., 2008). A staging system is used in humans that is of
essential. Attempted excision of invasive thymomas can be
unrewarding owing to the invasion of vital structures; how- prognostic value. In non-invasive forms, the rate of recur-
rent disease is approximately 2%, compared with 20% for
ever, the slow-growing nature of these tumours means that
incomplete excision may be palliative for some time. invasive forms that are successfully excised at surgery
(Monden et al., 1984, 1985). In dogs, myasthenia gravis
Cranial caval replacement with a jugular autograft has been
described in dogs to achieve en bloc tumour excision when and megaoesophagus are the most important prognostic
the cranial vena cava has been invaded. Cytoreductive sur- indicators. Dogs with non-invasive thymoma and no para-
gery such as this may be palliative for a prolonged period neoplastic megaoesophagus appear to have a good
of time. Postoperative monitoring by physical examination prognosis for survival, with median survival times of
and thoracic radiography every 3–6 months is recom- approximately 2 years (635–790 days) being reported (Zitz
mended, to detect tumour recurrence or development of et al., 2008; Robat et al., 2013). Invasive thymoma and the
myasthenia gravis and secondary megaoesophagus. presence of megaoesophagus carry a grave prognosis,
with high postoperative morbidity and mortality in several
studies (Bellah et al., 1983; Atwater et al., 1994; Scherrer et
al., 2008; Zitz et al., 2008). A more recent study in dogs
suggested that the presence of hypercalcaemia, myas-
thenia gravis or megaoesophagus did not influence
survival, but pathological grade based on the Masaoka–
Koga staging system did (Robat et al., 2013).
In humans, resolution of myasthenia gravis following
thymectomy occurs in approximately 30% of patients,
with significant improvement in approximately 60–70%
(Drachman, 1994). The response of acquired myasthenia
gravis to thymectomy in dogs and cats is poorly described,
with single reports of persistent or resolved disease follow-
ing surgery (Gores et al., 1994; Lainesse et al., 1996). The
development of myasthenia gravis following excision of thy-
moma has been observed in both humans and dogs. The
pathogenesis of acquired myasthenia gravis in thymoma
patients is not understood, but appears to involve immune
dysfunction and the development of autoantibodies to ace-
tylcholine receptors (Garlepp et al., 1984; Paciello et al.,
Intraoperative photograph of the cat in Figures 16.5, 16.6 and 2003). Other immune-mediated diseases that have been
16.11
16.10. A midline sternotomy has been performed to gain associated with thymoma in dogs and cats include poly-
access to the thorax and tumour.
myositis, granular cell proliferation and immune-mediated
skin disease in the dog, and myositis, acute moist derma-
titis, pemphigus foliaceus, superficial necrolytic dermatitis
Pericardium and hypogammaglobulinaemia in cats (Willard et al., 1980;
Carpenter and Holzworth, 1982; Day, 1997; Forster-Van
Hijfte et al., 1997; Smits and Reid, 2003). How these
diseases respond to excision of the thymoma is unknown.
Non-thymic tumours are occasionally seen in associa-
tion with thymoma in dogs, cats and humans. Up to 10%
of human patients with thymoma may have additional non-
thymic neoplasia, and seven of 22, five of 23 and 31 of 116
dogs with thymoma in three separate retrospective studies
had additional neoplasms (Bellah et al., 1983; Atwater et
al., 1994; Robat et al., 2013). Lymphoma and primary lung
tumours are the most commonly reported concomitant
tumours in dogs and cats. A failure of thymus-dependent
immune surveillance is thought to be responsible for the
development of these concomitant tumours. This fact
means that great care should be taken when evaluating
animals with thymoma both before and during surgery.
Gross anatomy of the thymoma removed from the cat in
16.12 Radiation therapy has been used as an adjunctive
Figures 16.5, 16.6, 16.10 and 16.11. This thymoma was not
invasive. Note how the mass has conformed to the shape of the therapy prior to or post surgery or as a solitary protocol in
surrounding thoracic wall and adjacent heart. The pericardium has been both dogs and cats. In one study of 17 dogs and seven
removed en bloc with the tumour. cats complete resolution of the tumour was rare, although

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