ABSTRACT: A nine-year-old female Boxer dog presented with a large infected mast cell tumour |MCT| on her left hock. Diagnosis confirmation and staging occurred prior to medical management to shrink the mass lesion, followed by surgical resection, Masitimb was then administered as follow-up treatment that has successfully prevented tumour re-occurrence for the last six months.

Case study

A nine-year-old female Boxer dog presented with what appeared to be a large infected tumour on the lateral aspect of her left hock (Figures 1 & 2). The mass had been present for a couple of months and had become infected. Fine-needle aspirates indicated the swelling to be a mast cell tumour (MCT) and antibiotics were helping to resolve the infection quickly.

Figure 1: The patient – a nine-year-old female Boxer

Figure 2: The suspected tumour on the lateral aspect of the left hind leg

At presentation, the patient was bright, alert, responsive and in fairly good body condition (body weight 28kg, condition score 3/5). Clinical examination revealed the mass to be raised, erythematous and approximately 5cm by 3cm in size. The popliteal lymph node was slightly enlarged, but the remainder of the examination was unremarkable.

The patient was admitted and blood samples were taken. Haematology and biochemistry were unremarkable. Under light sedation, abdominal ultrasound was performed. This revealed slightly enlarged left medial iliac lymph nodes. The remainder of the abdominal ultrasound was unremarkable. Fine- needle aspirates were also taken from the mass and left popliteal lymph node and found to contain a reasonable number of neoplastic malignant mast cells.

Initial treatment

At this stage, surgical resection could not be considered, not only because of the tumour size and position but also because of the metastatic lymph node involvement. After discussion with the owners, she was discharged to commence treatment with chemotherapy of chlorambucil (Leukeran, Glaxo Smith Kline) and prednisolone (Prednicare, Animalcare). The aim was to reduce the size of the mass and allow surgical resection with follow-up radiotherapy, as  at this stage it seemed likely that the mass was a medium grade MCT (Table 1).

The patient vomited occasionally whilst taking chlorambucil; but the mass reduced significantly in size. At ‘revisit’, the reduction of the tumour was such that it was freely mobile and surgical resection possible. The mass was removed with deep fascial and one centimetre margins that healed very well.

Unfortunately, however, histopathology of the mass revealed that the MCT was high grade, rather than the lower grade MCT usually seen in Boxers.

Response to treatment

Removal of the patients sutures was delayed for seven days, as tissue at the cranial edge of the wound was slightly puffy. She therefore returned a week later and at examination she was very bright. The wound had healed well, so skin sutures were removed.

Under light sedation, abdominal ultrasound was performed, revealing the left medial iliac lymph node to be smaller in size than seen previously, although one small splenic nodule was present. This was aspirated and there was no evidence that mast cells were present. This was, therefore, consistent with nodular hyperplasia.

Follow-up treatment

After a long discussion regarding follow- up chemotherapy, it was decided that the patient would start on masitinib (Masivet, AB Science) at a dose rate of 300mg SID. Masitinib belongs to a class of medications that have an anti-cancer action often prescribed to treat MCT It works as a protein-tyrosine kinase inhibitor targeting the cell receptors responsible for proliferation, directly addressing the origin of spread.

A mutating mast cell has been found to demonstrate excessive action of c-kit, stimulating uncontrollable division.

By blocking thyrosine kinase enzymes, the c-kit receptors found on the cell surface are inhibited.

It was considered that mastinib administration in this case should, therefore, help reduce the number of residual neoplastic mast cells by controlling cell division, thus preventing tumour progression. This option was chosen on the basis that it should be efficacious and, given the vomiting seen with chlorambucil, should be safe for the owners to administer.

Another option that has since become available is toceranib phosphate (Palladia, Pfizer). Like masitinib, toceranib phosphate is licensed for the treatment of MCT in dogs.

Following two weeks of therapy, the patient returned in order for the clinical team to monitor the response to treatment and assess for possible side effects (Table 2), none of which had been noted and the surgical site was healing well.

The patient was briefly admitted for blood samples to be taken. Haematology was unremarkable. However, biochemistry revealed both urea and creatinine to be the top end of the normal range. A 25 per cent dose reduction in the masitinib was made, alternating a dosage of 300mg on one day followed by 150mg the next day.

On the sixth week of masitinib treatment, clinical examination revealed that there was a very small nodular area of approximately 3mm in diameter, at the caudal edge of the wound. This appeared intimately associated with the scar and had reduced significantly since she was last seen. The patient was still far too lively when conscious to aspirate this nodular area. She was admitted briefly for blood samples to be taken. Haematology was unremarkable and biochemistry revealed that the urea and creatinine levels were stable. Administration of masitinib was, therefore, continued as previously prescribed.

At 12 weeks of masitinib therapy, the very small nodular area at the caudal edge of the wound had reduced significantly in size and the popliteal lymph node appeared normal. Haematology was unremarkable and biochemistry revealed that the urea and creatinine levels were stable. Once again, administration of masitinib continued as before and an appointment was made to see her again in six weeks time with a plan to consider withdrawing the masitinib at this stage, it being six months after commencement of treatment.

Discussion relating to this case

Mast cell tumours form as a consequence of deregulation from normal mast cells in the skin. They originate from the bone marrow and migrate throughout the connective tissue of the body as a normal component of the immune system. Mast cells contain inflammatory mediators including histamine, heparin and platelet-activating factors and play a
key role in wound healing, acute and chronic inflammatory and allergic responses.

MCTs are the most common cutaneous malignant neoplasm, accounting for 20 per cent of all cutaneous tumours seen in dogs. They are aggressive and nietastasise readily to local lymph nodes, liver, spleen and bone marrow.

Surgery is the treatment of choice, with an antihistamine administered at induction to protect against anaphylaxis as a consequence of histamine release. Wide margins of 2-3cm are required as cells frequently spread beyond margins of the visible lesion. Complete removal of low to medium grade MCT carries a good prognosis; although low grade MCT of the gastrointestinal tract, paw and muzzle have a guarded prognosis.

A poor prognosis is warranted if there is lymph node involvement, mastocytosis or if the MCT is high grade. The median survival times of low to medium grade MCT are reported in dogs to be over 1,300 days in one study, in comparison with 278 days for high grade tumours.

Medical management is often prescribed as an adjunctive therapy for incomplete resections or cytoreductive for reducing large tumours prior to resection. It is also administered palliatively if surgery is not an option.

Various combinations of chemotherapeutic agents can be considered and radiotherapy may also be an option to further reduce focal lesions. In this case, masitinib was a favoured choice, because it was licensed to treat MCT and continued to demonstrate effectiveness.

Author

Anya Owen Dip AVN HE CVN (SA)RVN

Anya is the head medicine nurse working at Dick White Referrals veterinary specialist centre in Suffolk, where she has worked for the last five years. With a particular interest in cardiology, oncology and anaesthesia, she enjoys all aspects of referral nursing, striving to provide the highest level of nursing care.

To cite this article use either

DOI 10.1111/j.2045-0648.2011.00087.x or Veterinary Nursing Journal Vol 26 pp 319-321

Suggested reading

FOALE. R and DEMETRIOU. J. 120101 Solutions in Veterinary Practice: Smatl Animal Oncology, Ed F Nmd, Elsevier Saunders.

DODSON. J M and LASCELLES. B D 120031 BSAVA Manual ol Canine and Feline Oncology 2nd Edn British Small Animal Veterinary Association. Gloucester

Veterinary Nursing Journal • VOL 26 • September 2011 •