‘The Great Pretender’ is a term often given to canine mast cell tumours (MCT) due to their ability to mimic a variety of conditions. Sometimes they are rapidly growing, erythematous masses which clearly warrant further investigation, but at other times they can look and feel like a simple lipoma which can make it easy to assume these masses are completely innocent. For this reason it is essential, if at all possible, to perform a fine needle aspirate on all masses encountered within the consulting room.

However, despite the diagnosis of the MCT being relatively simple, it is still sometimes difficult to know whether a MCT needs to be staged and what level of treatment is required to offer the best outcome for a patient. Is surgery alone adequate or is there a requirement for chemotherapy and radiotherapy? All these questions and more were answered in last week’s webinar led by Dr Thamm who delivered clear up to date evidence based advice on the diagnosis and management of MCTs.

To stage or not stage was one of the questions answered within last week’s webinar with Dr Thamm explaining that it is not always necessary to stage every single MCT. This decision can be made once a number of factors have been taken into account. Firstly Dr Thamm will always try to perform an FNA on the lymph node most local to the MCT. If there is no sign of spread to this local lymph node, then staging can be skipped but only if there are no other negative prognostic indicators. These include the presence of MCTs on mucous membranes which are notoriously more likely to be high grade in nature. Rapid growth of a tumour is another negative prognostic indicator and attaining a good history from the owner is key to determining speed of growth.

Dr Thamm also explained that solitary mast cell tumours are rarely associated with clinical signs which, for example, may be seen with gastric ulceration. Consequently any dogs demonstrating clinical signs other than the presence of a mass should always be staged by further investigation. Staging involves performing full bloods to check for any organ involvement and thoracic X-rays which are usually only of benefit to assess local lymph node involvement. Abdominal imaging is also very useful in assessing for lymph node, spleen and liver involvement although Dr Thamm only advises performing FNAs if abnormalities are found within any of these organs and not if they look normal.

Aggressive surgery is usually sufficient therapy for the treatment of low and intermediate grade MCTS and Dr Thamm advises to, if possible, always take a 3cm margin around the tumour in all fascial planes. There is however an alternative approach where the size of the margin taken is equal to the diameter of the tumour (with a 3cm margin as a maximum). According to a study utilising this technique 85% of MCTs were completely excised. However most of these tumours were only low to intermediate grade and as the grade of a tumour can only be ascertained at histopathology, Dr Thamm still advises using 3cm wide margins during surgical excision.

The decision on post-operative management regimes can be another source of confusion and Dr Thamm provided a table which gave very clear advice on appropriate treatments according to the grade of tumour and the success of the surgery. This table showed that a grade 1-2 tumour with clean margins requires no further treatment. However a grade 1-2 tumour with dirty margins needs repeat surgery or radiotherapy. A grade 3 tumour with clean margins warrants chemotherapy and a grade three tumour with dirty margins warrants repeat surgery or radiotherapy and chemotherapy.

Chemotherapy consists of the use of prednisolone and vinblastine which has relatively mild and self-limiting side effects. Dr Thamm also discussed the use of c-KIT inhibitors with 20-40% of MCTs having a mutation of the c-KIT gene which leads to constitutive activation of mast cells. C-KIT inhibitor drugs include masitinib and toceranib and can also be used in combination with radiotherapy and chemotherapy although the dose of all the chemotherapy drugs will need to be reduced significantly.

This webinar provides an excellent foundation from which an evidence-based approach can be devised for the management of patients with mast cell tumours with Dr Thamm offering clear advice on the steps which need to be taken in order to make the right decisions for each individual case.

For those of you who need mast cell tumours ‘demystifying’ or for those of you who just want to keep up to date with the current thinking on the management of these ‘great pretenders’ then last week’s webinar should not be missed.