Downs Veterinary Referrals

Mast cell tumours (MCT) are among the most common canine cutaneous neoplasms seen in practice.

fig.1 - Fine needle aspiration of a solitary cutaneous lesion on the flank of a Rhodesian Ridgeback.

17-21% of all skin tumours are diagnosed as MCT.  There is no single, pathognomonic presentation for MCT.  Therefore, they should be considered as a possible differential diagnosis with any cutaneous nodule.

Breed predispositions have been reported, with more than 50% of MCT presenting in Boxers and Boston Terriers. No gender predilection is reported. The mean age at presentation is 7.5-9 years.  MCT mostly affect the trunk, followed by the extremities and the head and neck.  They are more commonly found towards the caudal part of the trunk than cranially.  They commonly present as solitary cutaneous lesions though 5-25% of dogs are affected by multiple lesions.

No single factor has been identified as a consistent prognostic indicator in dogs with mast cell disease.  However, MCT located in the nail bed, oral cavity, muzzle, inguinal region, preputial region, perineal region or at the mucocutaneous junction are associated with a worse prognosis than masses at other sites.  Ulceration, erythema or pruritus associated with the lesion is also considered a poor prognostic indicator.

Systemic clinical signs associated with the visceral form of the disease such as anorexia, vomiting, haematochezia, melaena, anaemia, abdominal pain, GI perforation and peritonitis indicate a poor prognosis.  35-83% of dogs with mast cell tumours demonstrated associated visceral lesions at necropsy.  These lesions are caused by vascular damage and H2 receptor stimulation resulting in increased gastric acid production.

MCT in Boxers tend to present at a younger age, with a lower histological grade than other breeds and are associated with a better prognosis.  Rapidly growing masses tend to be associated with a poorer prognosis as do masses that are large in size on presentation.

Metastatic disease

fig.2 - A well-differentiated MCT identified on cytology (grading was performed on a subsequent, histological sample).

Well-differentiated MCT have a metastatic rate of less than 10%. Intermediate grade tumours are of low to moderate metastatic potential.  55-96% of undifferentiated tumours metastasise.  The variability in this figure may be due to the subjective nature of MCT grading criteria.  Disseminated mastocytosis is usually preceded by an undifferentiated, primary cutaneous mast cell tumour.  Mast cell tumours initially metastasise to local lymph nodes, then the spleen, liver or other viscera.  Lung involvement is rare in mast cell disease.

Paraneoplastic syndromes and complications of granule release

• Delayed wound healing – due to proteolytic enzyme and vasoactive amine release.
• Bleeding – likely to be due to coagulation defects caused by heparin release.  Rarely uncontrollable.
• GI ulceration
• Collapse – due to massive histamine release from neoplastic cells.  This is a potential complication of manipulation and/ or surgery.

Diagnostic work-up

The aims of this process are to establish a definitive diagnosis, to provide clinical staging of the disease and to document paraneoplastic clinical signs.

Tumour cytology

• This should always be performed prior to surgery.

FNA (see fig 1) yields a diagnosis in 92-96%

Figure 2 shows a discrete, round cell population with a moderate amount of cytoplasm containing purplish-red cytoplasmic granules of variable number and size.  The cells have ovoid nuclei which may be masked by granules.

Further diagnostic work-up

Minimum pre-operative work-up for staging is FNA of draining lymph nodes even if they are normal in size, and abdominal ultrasound to include the sublumbar lymph nodes.  Suspicious visceral lesions should be aspirated. Thoracic radiographs are seldom indicated, unless looking for concurrent disease in older animals, as metastasis to this site is rare.  The value of screening of the buffy coat is disputed as mastocytaemia is more common in inflammatory conditions than in MCT.  Bone marrow biopsy is no longer recommended for routine staging if mast cell disease is not evident in regional lymph nodes or abdominal organs. Visceral MCT carries a grave prognosis and the presence of neoplastic mast cells in bone marrow rarely alters treatment decisions.

Staging

WHO staging system

• Stages 0 and I have a better prognosis than higher stage disease.
• No significant difference has been reported in single v multiple masses of the same histological grade.  As a result, it has been suggested that each mass should be staged individually to avoid unnecessarily aggressive treatment based on the limitations of the WHO staging system.
• Up to 44% of dogs cured of previous MCT will develop de novo MCTs which require individual staging and grading.

Histology

fig.3 - A grade I, well-differentiated MCT. These masses are well circumscribed but not encapsulated. Cells are round with a central, small nucleus. Abundant, highly granulated cytoplasm is seen. MCT of this grade are usually confined to the dermis but may also invade the subcutaneous tissue.

fig.4 - A grade III MCT. These masses are poorly circumscribed and poorly differentiated. The cells are highly pleomorphic and can be confused with the appearance of other round cell tumours. The cells have large, irregularly shaped nuclei and contain numerous mitotic figures.

Grading is based on histology, not cytology.  Margin assessment should also be performed on excisional biopsy.

Patnaik et al (1984) described the histopathological classification most frequently used to differentiate between well-differentiated, intermediate grade and undifferentiated MCT.  Grade does not correlate well with behaviour in intermediate grade masses as they may behave in a benign manner, recur or metastasize.  More than 40% of all canine MCT are described as intermediate grade.  The subjective parameters used for grading result in variation between pathologists using identical grading systems.

Intermediate (Grade II) MCT

These masses are less well circumscribed and contain closely packed cells with distinct cell borders.  They have an increased nuclear:cytoplasmic ratio and fewer granules. Small numbers of mitotic figures are seen.

Grading based on markers

Mitotic Index – marker of morphology

• Studies have demonstrated a prognostic value of MI in canine MCT.
• Median survival for MI<5=70 months v MI>5=2 months regardless of histological grade.
• Difficult to assess in highly granulated MCT.
• Objective measure with strong prediction of prognosis with regard to survival time.
• Impact of MI on rate of recurrence is controversial.

AgNOR – marker of proliferation. 

• Indicated regions in the nucleus associated with proteins that bind silver molecules that can be visualised by light microscopy using a silver-based immunohistochemical stain.

• Quantity of AgNORs per nucleus is proportional to the rate of cell doubling time in vitro and rate of tumour growth in vivo.
• Higher AgNOR counts in MCT are associated with increased mortality, local recurrence and metastasis.

Ki67 – marker of proliferation.

• Nuclear protein expressed in all active phases of the cell cycle but not present in quiescent cells.
• The number of Ki67 positive cells is used to determine the relative number of cells actively involved in the cell cycle; the growth fraction.
• High Ki67 is associated with increased mortality, increased rate of local recurrence and metastasis.
• Ki67 is considered better than AgNOR for identifying MCT associated with decreased survival.
• AgNOR is considered superior for identifying MCT with a decreased disease free interval.

Treatment

Visceral metastatic disease

Local control should be accompanied by systemic treatment

• Cytotoxic chemotherapy

Þ Prednisolone/Vinblastine

Þ Prednisolone/Vinblastine/Cyclophosphamide

Þ Prednisolone/Vinblastine/Lomustine

Þ Comparative efficacy studies are still

pending.

• Ancillary treatment to decrease systemic side- effects of MCT granule release

Þ H1 and H2 blockers

Þ Proton pump inhibitors

Þ Sucralfate in cases of GI ulceration

• Radiation therapy

Recommended for post-op residuals and microscopic/subclinical disease if further resection is not possible. It has also been employed pre-operatively to reduce tumour size.

• Masitinib (Mastivet)
  • Novel, targeted therapy for use in mast cell tumours and other inflammatory diseases.
  • Targets and inhibits a limited number of key kinases including c-Kit.
  • c-Kit, a mast/stem cell growth factor receptor, demonstrates mutations in 20-30% of canine mast cell tumours. This mutation correlates with a higher histological grade.
  • When used as first-line therapy in dogs (grade II or III cutaneous MCT) with both mutant and wild-type forms of c-Kit, masitinib significantly increased time to progression (TTP) of mast cell tumours when compared to a placebo.
  • Masitinib is a p-glycoprotein substrate and the effectiveness of the drug can be compromised by activation of the MDR-1 gene. Dogs that developed resistance to other chemotherapeutic agents did benefit from masitinib treatment. TTP was only increased by masitinib when the MCT expressed c-Kit mutation.
  • Utilisation of masitinib as part of a multidrug chemotherapy protocol is not yet recommended due to an absence of evidence and the similar toxicity profiles of masitinib and other chemotherapeutic agents.
  • Care should be taken in incorporating masitinib into protocols containing cimetidine and vinblastine as competition between these drugs on cytochrome P450 enzymes could reduce metabolic clearance of these drugs resulting in toxicity.
  • Side effects include vomiting and diarrhoea of mild-moderate intensity and transitory nature. Improved tolerance of the drug is possible over long-term treatment regimens. Protein loss has also been reported resulting in hypoalbuminaemia from which dogs recover and do not relapse after discontinuation of masitinib therapy.
  • Contraindications:
    • Þ Renal dysfunction
    • Þ Hepatic dysfunction
    • Þ Anaemia
    • Þ Neutropaenia
    • Þ Administration to pregnant or lactating bitches
    • Þ Dogs less than 6-months of age or less than 3.4kg body weight.

Further reading

1. Thamm DH, Vail DM (2007) Mast Cell Tumors in Withrow & MacEwan’s Small Animal Clinical Oncology p402-424 Saunders Elsevier, Missouri.

2. Welle MM, Bley CR, Howard J, Rufenacht S (2008) Canine mast cell tumours: a review of the pathogenesis, clinical features, pathology and treatment. Vet Dermatol 19(6): 321-39.

3. Hahn KA, Ogilvie G, Rusk T, Devauchelle P, Leblanc A, Legendre A, Powers B, Leventhal PS, Kinet JP, Palmerini F, Dubreuil P, Moussy A, Hermine O (2008). Masitinib is safe and effective for the treatment of canine mast cell tumours. J Vet Intern Med. 22(6):1301-9.