Mast cell tumours are a rare skin disease of cats, but the second most common feline skin tumours (following basal cell tumours). Mastocytaemia in cats appears exclusively associated with mast cell tumours^[1].

There are two forms of mast cell tumours; a mastocytic form that histologically resembles normal mast cells and a histiocytic cutaneous form that histologically resembles histiocytic mast cells. Mast cell tumours may be cutaneous or visceral^[2].

• Mastocytic form – most commonly affects cats with an average age of 10 years, and Siamese cats may be predisposed. However, tumours of this form have been reported in all breeds.
• Histiocytic cutaneous form – affects young Siamese cats (6 week old – 4 years)
• Histiocytic visceral form – usually metastatic and the third most common intestinal tumour after lymphoma/lymphosarcoma and adenocarcinoma.

Mast cells are found throughout the body but are most numerous in the skin and the vascularised submucosae of the gut and respiratory tract. Mast cells are of mesenchymal origin. Undifferentiated precursor cells, of bone marrow origin, migrate through vascular walls and differentiate in situ. The life span of mast cells is of the order of months. Cutaneous mast cells are located in the dermis and hypodermis. In response to stimulation, mast cells in cats may migrate into the epidermis. Mast cells are an important part of the skin immune system. The main function of mast cells is the production of inflammatory mediators (esp, histamine). They also play an important part in the regulation of wound healing. Main trigger for mast cell activation/degranulation is cross-linking of surface bound IgE. Other triggers include neuropeptides from nerve endings (e.g. substance-P), histamine-releasing factors and interleukin-1 from inflammatory cells, cytokines, physical irritants and thrombin from vascular endothelium^[3].

Mast cells produce pre-formed and synthesised mediators. Pre-formed products are located within the mast cell granules and are ready for immediate release. Synthesised factors are one of the major mechanisms of late-phase inflammatory reactions. Heparin, a synthesised mast cell product, is important for inhibition of clotting once injured blood vessels have been sealed. Based on the types of enzymes produced, it is thought there are different populations of mast cells. It is not clear if this has some relation to the difference in behaviour of mast cell tumours.

 

Clinical signs

Clinical signs are often missed by the carer of the cat, but clinical presentation is usually for non-pathognomonic signs such as weight loss, intermittent anorexia, vomiting, lethargy and depression.

In visceral-splenic mast cell tumours, physical examination usually reveals a massive diffusely enlarged spleen. Abdominal effusion may also be noted^[4].

 

Diagnosis

Cutaneous mast cell tumours are usually benign but can occasionally metastasize to the regional lymph nodes, spleen, liver, bone marrow and rarely, lung. Cutaneous tumours occur more commonly on the head and neck, may be pruritic or non-pruritic, alopecic nodules general 2-3cm in diameter and have a variety of appearances. The histological grading system used in dogs is not valid in cats, although mastocytic mast cell tumours occur in either a compact (50-90% of cases) or diffuse histological form. The compact form has a more benign behaviour, while the diffuse form has more anaplastic and malignant behaviour^[5].

Histological confirmation can be challenging because mast cells comprise only 20% of the lesion. Eosinophils are not a feature of feline mast cell tumours.

Visceral mast cell-like tumours occur commonly in the pyloric sphincter, ileocecocolic junction or colon and small intestine. Twenty five percent of these cases also had lymph node involvement. The lesions were characterized by eosinophilic inflammation, large reactive fibroblasts, and trabeculae of dense collagen similar to mast cell tumours. However, intralesional bacteria were identified in 56% of the cases overall (Clostridium perfringens and E. coli) and all of the ileocecocolic junction and colon lesions. Fifty-eight percent of cats tested had peripheral eosinophilia. Cats treated with prednisone had a significantly longer survival time than those receiving other treatments. This appears to be a unique fibroblastic response of the feline gastrointestinal tract to eosinophilic inflammation that in some cases is associated with bacteria. These lesions are often grossly and sometimes histologically mistaken for neoplasia^[6].

Differential diagnoses for cutaneous mast cell tumours include SCC, melanoma, basal cell tumour, fibrosarcoma, cutaneous hemangioma/hemangiosarcoma, eosinophilic granuloma complex, panniculitis and sebaceous adenoma.

 

Treatment

Primary treatment of mast cell tumours involve surgical excision of primary tumour, splenectomy in cases of splenic involvement.

The effectiveness of chemotherapy (including corticosteroids, chlorambucil, vinca alkaloids) and radiation therapy have not been extensively evaluated. Chemotherapy has had poor response rates without improvement in survival time. Radiation therapy for cats with solitary cutaneous mast cell tumours has had response rates up to 60%1.

Most mast cell tumours are cured with complete surgical excision. Local recurrence rates are reported at less than 36% (typically within 6 months), and metastatic rates are reported at less than 22%. Cats undergoing splenectomy for splenic mast cell tumours have a median survival time of 12-19 months, and less than 6 months without splenectomy. Intestinal mast cell tumours have the poorest survival time of less than 4 months.

Novel treatment using gene therapy with Imatinib mesylate has shown promise in cats with mast cell tumours.

 

References

1. ↑ Skeldon, NC et al (2010) Mastocytaemia in cats: prevalence, detection and quantification methods, haematological associations and potential implications in 30 cats with mast cell tumours. JFMS 12:960-966
2. ↑ August, JR (2010) Consultations in feline internal medicine. Vol 6. Elsevier Saunders, Philadelphia. p:697
3. ↑ Norsworthy, GD., et al (2003) The feline patient: essentials of diagnosis and treatment. 2nd edition. Lippincott, Williams & Wilkins
4. ↑ Buerger, RG & Scott, DW (1987) Cutaneous mast cell neoplasia in cats: 14 cases (1975-1985). J Am Vet Med Assoc 190:1440
5. ↑ Hilton, R. (2005) The Veterinarian
6. ↑ Craig, LE, Hardam, EE., Hertzke, DM., Flatland, B., Rohrbach, BW., & Moore, RR (2009) Feline Gastrointestinal Eosinophilic Sclerosing Fibroplasia Vet Pathol 46:63-70