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A dog with circulating “atypical” lymphocytes; Is it a lymphoma or a leukaemia?

18th October 2017

Receiving a haematology report which shows that a dog has “atypical lymphoid cells” in circulation is always a concern. Key differential diagnoses include stage V lymphoma or acute lymphoid leukaemia. Deciding which disease you have is very important because stage V lymphoma should respond favourably to treatment and many studies show that involvement of the bone marrow makes no difference in the lymphoma’s prognosis. Acute leukaemia on the other hand, is characterised by a poor response to treatment and remissions are not durable. Before deciding on whether to treat the disease, our clients usually appreciate being advised on the chances of success!

So how can we tell which is occurring? First the dog him/herself can give you some clues. Lymphoma which involves the bone marrow often also involves grossly enlarged peripheral lymph nodes and/or organomegaly, the count of abnormal circulating cells is often low, and cytopaenias are often mild or absent. Dogs with acute leukaemia usually have prominent lymph nodes, but rarely exhibit the gross lymphadenomegaly of lymphoma cases, cytopaenias are usually more severe and the count of atypical cells is usually high. These guidelines serve as a rule of thumb for what to expect but cannot be relied upon completely.

A more categoric distinction can be achieved by sending blood for flow cytometry to see if the cancer cells exhibit the stem cell marker “CD34.” Circulating cells that are CD34 positive can be considered to be representative of an acute leukaemia whereas those which are negative are considered to represent lymphoma. Flow cytometric evaluation of blood then is probably the most definitive test we have, but beware that results can sometimes be erroneous; cancer cells, by definition, have escaped the body’s control and so there are no rules as to which antigens they should or shouldn’t express!

Flow cytometry is available in the UK and turnaround is often rapid. Please check with the lab concerned on how they would like the samples taken and shipped.

If you have a sick dog with a neoplastic lymphocytosis and you’d like advice on management of the case please feel free to contact the Highcroft Oncology Service on 01274 838473.

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