Feline mast cell tumours
Mast cell tumours are the second most common feline skin tumours (following basal cell tumours). There are two forms: 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 visceral1.
| 1) 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 |
| 2) Histiocytic cutaneous form |
affects young Siamese cats (6 week old - 4 years). |
The biology of mast cells
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 (SIS). 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 and thrombin from vascular endothelium
- Complement
- Feedback from fibroblast cytokines
- Physical irritants
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 tumours2.
Diagnosis
Cutaneous mast cell tumours are usually benign but can occasionally metastasise 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 nonpruritic, 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.
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Mast cell tumours on the head of a cat |
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| Fig. 1A. Cutaneous mast cell tumor (well differentiated), dog, Wright-Leishman stain. The mast cells have numerous purple granules that partially obscure nuclear morphology. | Fig. 1B. Cutaneous mast cell tumor (well differentiated), dog, Wright-Leishman stain. Numerous mast cells are present. Numerous purple cytoplasmic granules obscure nuclear detail. Many mast cells also have poorly stained (pale blue) nuclei. |
Histological confirmation can be challenging because mast cells comprise only 20% of the lesion. Eosinophils are not a feature of feline mast cell tumours. Differential diagnoses for cutaneous mast cell tumours include SCC, melanoma, basal cell tumour, fibrosarcoma, cutaneous hemangioma/hemangiosarcoma, eosinophilic granuloma complex, panniculitis and sebaceous adenomas.
Mast cell tumours are the third most common intestinal tumour after lymphoma/lymphosarcoma and adenocarcinoma.
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.
Prognosis
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 months1.
1. Norsworthy, GD., et al. (2003) The feline patient: essentials of diagnosis and treatment. 2nd edition. Lippincott, Williams & Wilkins
2. Hilton, R. (2005) The Veterinarian http://www.theveterinarian.com.au/clinicalreview/article638.asp