Companion May 2012 - BSAVA

14 | companion
Clinical conundrum
What is your interpretation of the
cytology? (Figure 2)
Within a background of fresh blood there is a
scattered heterogenous population of lymphoid
cells, dominated by small lymphocytes and a
single cluster of larger atypical epithelial cells.
Diagnosis – Thymoma.
Differentiation between thymoma and lymphoma
cytologically is challenging as both will display
prominent lymphoid populations, normal and
heterogenous in the former, homogenous and
neoplastic in the latter. The neoplastic component of a
thymoma is epithelial and only when this is identified,
as in this case, can a cytologist be confident of a
diagnosis of thymoma. Mast cells can be identified in
upto 85% of cytological samples from thymomas, and
this may also aid differentiation. Often histological
examination is required to obtain a definitive
diagnosis, sometimes even immunohistochemistry. In
this case Tru-cut biopsy had been planned had
cytological examination not immediately yielded a
definitive diagnosis.
Are there any clinical signs of any of the
paraneoplastic syndromes associated
with canine thymoma in this case?
The paraneoplastic syndromes associated with
thymoma are listed below. In this case, there were no
clinical signs which were not immediately attributable
to the mass effect of the neoplasia. This is of relevance
as, for example, the presence of megaoesophagus
due to myasthenia gravis, is a strong negative
prognostic indicator and such patients may be poor
treatment candidates.
■■ Myaesthenia gravis
■■ Paraneoplastic dermatitis
■■ Paraneoplastic hypercalcaemia
■■ Cranial vena cava syndrome
■■ Horners Syndrome
What treatment options are available for
canine thymoma?
Chemotherapy – Thymoma are poorly responsive to
cytotoxic agents, although some reduction in size may
be seen. However it is unclear if size reduction is due
to an anti-neoplastic effect or solely because the
cytotoxic and/or corticosteroid protocols reduce the
non neoplastic lymphoid content of the thymoma.
Chemotherapy is indicated if other treatment
modalities are unsuitable or unavailable and may offer
some symptomatic relief associated with mass effect.
Surgery – Surgical excision, if feasible, can be
curative and the clinical signs associated with the
mass effect will be relieved. However the feasibility of
the surgery depends on the invasiveness of the mass,
its association with the phrenic nerves and adhesions
to vascular structures. When full excision is achieved
recurrence is uncommon and survival times in excess
of 2 years are expected.
Radiotherapy – If surgical excision is not possible due
to the invasiveness of the thymoma, radiotherapy can
be considered. Three quarters of thymoma treated in
this way can be expected to shrink to some degree,
occasionally completely. Survival times typically
average nine months.
As is implied by the above discussion, whether the
thymoma is invasive or not determines the treatment
strategy employed. Typically half of cases will be
classed as non invasive (well encapsulated) and
further information regarding this distinction is gained
using advanced imaging. CT provides information on
the definitive margins of the mass, its size and extent
of the mass effect as well as allowing evaluation of
invasion into surrounding structures such as the cranial
vena cava, thoracic wall and pericardium. CT offers
considerable advantages over thoracic radiography
allowing better distinction between solid, lipid, cystic,
mineralisation and vascular structures. CT is also more
sensitive in identifying pulmonary metastases in the
rare cases of thymic carcinoma.
However it can be easy to overestimate invasion
into associated structures on CT alone and often the
true extent of invasion or local adhesions is only
apparent during surgery. As such, images should be
evaluated by a surgeon and a radiologist and a
consensus of opinion reached. In short, whilst CT
evaluation can identify those patients in which excision
is clearly indicated or those in which it appears
impossible, there will still be a proportion of individuals
in which it will only become clear at exploratory
thoracotomy if resection is feasible.
Should resection not be possible then the
information gained at CT will be useful in the planning
of radiotherapy.