Mast cell tumours (MCTs) is frequently seen in some breeds of dogs and is extremely important in the field of veterinary oncology. It represents the third most common tumour subtype, and is the most common malignant skin tumour in dogs, making up approximately 20% of canine skin tumors¹. Some dog breeds commonly are predisposed to MCT's are Boxers, Bull Terriers, Golden Retriever, Labrador Retriever, and Dachshund, while dog breeds like German Shepherd, Chihuahua, Poodle, Yorkshire Terrier and Cocker Spaniel are at a lower risk of MCT development². Canine MCTs can be of different sizes,may be delimited, elevated, firm, soft, pruritic, ulcerated, erythematous with or without invasion of the subcutaneous tissue. The findings suggestthat about 50% of canine MCTs develop in the trunk, perineum, and inguino-genital regions, 40% occur in the limbs, and 10% in the head and neck³. Also, the metastatic movements frequently involvethe lymph nodes, skin, spleen, and liver and less frequently the lungs. These tumours exfoliate high numbers of typical cells containing large numbers of small, round, purple granules, making diagnosis relatively easy. Thus, cyto-pathological technique is themost routinely used, fast and cost effective method used to diagnose MCTs. Ultrasonography and computed tomography (CT) are imaging technique that has been used more frequently in recent years as a more sensitive tool in identifying metastatic lesions in different neoplasms⁴.

MCTs have variable clinical presentation, since their biological behaviour is variable. In general,when MCTs are well-differentiated, they present a milder behaviour. In contrast, less-differentiated tumours have a more aggressive behaviour. In the current case, several clinical signs like rapid growth, pruritus, severe localised inflammation and infiltrative reactions, ulcerations, poor demarcation from adjacent tissues and satellite nodules in the affected dog was suggestive of MCTs exhibiting aggressive behaviour.

An adult male Labrador dog aged about 6 years was presented in the veterinary clinical complex with the history of a small gradually developing swelling in the left l thigh on medial aspect of tibia and femur for the last about 7 months with severe and generalized pruritus. The dog showed reduced appetite, vomiting tendency, weight loss and apathy. The lesion was progressive and with time, it has involved whole thigh region. The dog was being treated with different antibiotics but no appreciable improvement was noticed. As per history, previous symptomatic treatments like antiinflammatory, anti-histamine, antibiotics with suitable topical gel application on the concerned area was given to the dog with no substantial improvement. Also, acaricidal treatment was provided to rule out any parasitic infestation. All these efforts lead to temporarily relief only with mild reduction insize of growth and pruritus with increase in appetite. With due course, the limb lesions were worsening, spreading onwards and not responding to treatment anymore and the animal remained apathetic with lot of sufferings.

Physical examinations revealed apathetic dog with deteriorating body conditions with slight rise in body temperature. Dermatological examination showed that the affected area was markedly enlarged, firm and the surface was ulcerated, bleeding, necrosed and covered with purulent, haemorrhagic discharge. The swelling appear to be covering a wide area with severe inflammation including purulent myositis and cellulitis on the medial aspect of leftthigh (Fig. 1). There was alopecia and scaling with erosions and ulcers observed on the affected area. On careful examination, there was few palpable small firm masses found surrounding the primary lesions. Blood samples, skin scrapings, touch smear, fine needle aspirates from site and ultrasound guidedaspirates from adjoining inguinal lymph nodes were collected for cytological examination (Fig. 2). The tissue samples were collected for further histopathological procedure and preserved in 10% formalin. Ultrasonography was also performed to ascertain the extent of spread.

The haematological examination showed increase in number of neutrophils (76%) and eosinophils (10%) with marked leucocytosis (16*10³/mm³). The reduced value of haemoglobin (10.5 gm%) suggested mild anaemia. The biochemical values were within normal limits however,there was mild increase in the values of aspartate aminotransferase (65IU/L) and Alanine transaminase (145IU/L), suggesting mild liver disorders.

Ultrasound findings revealed that the swelling had poorly distinct margin and anechoic to hypoechoic area on affected muscle part (Fig. 3) while hypoechoic area on inguinal lymph node (Fig. 4). Also there was a very thick diffuse hyper-echogenic subcutaneous tissue with some fluid filled cavities on the affected part of the limb. Abdominal ultrasound showed increased size of lymph nodes with few hyperechoic foci, moderate hepatomegaly while other organs appeared almost normal.

Smears collected were stained with Giemsa stain. Cytological examination of the touch smear from necrotic areas showed mainly necrotic cells, degenerated neutrophils (pus cells), eosinophilic and neutrophilic inflammation with concurrent presence of different forms of bacterial population. Cytological evaluation of fine-needle aspirates comprised many pleomorphic mast cells with highly granulated cytoplasm (Fig. 5A and B). Some of the mast cells were de-granulated which might have been occurred during collection of aspirates and smearing it. Marked cellular atypia was observed within the mast cell population thus including anisokaryosis, anisocytosis, hyperchromatism, prominent nucleolation, bizarre nuclei and good number of mitotic figures.

The histopathology was done by paraffin embedding technique and sections of 5q were cut and routinely stained with H&E stain. The microscopic examination of stained slide revealed moderately pleomorphic neoplastic mast cells (anisokaryosis and anisocytosis), with round and pleomorphic nuclei having intracytoplasmic granulation of varying sizes which have extended into the subcutaneous and muscular tissue with discrete areas of edema, necrosis, and collagen hyalinization (Fig. 6). On an average one to two mitosis figures per high power field were also visible.

Based on the clinical behaviour and cyto-histopathological findings, the case was diagnosed as malignant mast cell tumour which was well authenticated by ultrasonography findings. The grading system proposed by Patnaik et at.⁵, is the most commonly used system for cutaneous histological classification of MCTs, and divides tumours into grades 1, 2, and 3. The findings suggests that the present case can be concluded to be a case of grade 2 mast cell tumour. However, more elaborative work was needed to be done for confirmation of its grade and thereafter, suggest treatment regime. Staining with toluidine blue and immunohistochemistry would have thrown better insights. Combining Ki-67 expression with toluidine blue staining is useful in differentiating MCTs from other lesions such as eosinophilic granuloma⁶.

Mast cell tumours of the skin can occur anywhere on the body and there is significant variation in their biological behaviour. They can appear as a raised lump or just under the skin. Similarly they may be red, ulcerated, or swollen. Degranulation of the histamine causes inflammatory pruritus and subsequent swelling of the surrounding tissue. The etiology of MCT has not been completely elucidated. However, it is postulated that the influence of chronic inflammation in the skin and exposure to irritating compound (allergens) can be inciting causes. Furthermore, the presence of mutations in the c-KIT gene (KIT) has been related to tumour development in MCT cases. This gene encodes a receptor tyrosine kinase that binds stem cell factor (SCF) in canine mast cells. Mutations drive uncontrolled cell survival and proliferation, which is related to MCT development and progression⁷.

The metastatic potential of MCTs varies according to its histopathological classification. In well-differentiated mast cell tumours, metastasis occurs in less than 10% cases;in moderately differentiated neoplasms, metastasis occurs in 5% to 22% cases and up to 55% to 95% metastasis occurs in poorly differentiated cases. Metastases occur mainly on regional lymph nodes and may later affect the spleen, liver, and other organs. Patients with mast cell tumours of any degree and who have regional lymph node involvement in general have a poorer prognosis⁸. The main therapeutic measures indicated in the cases of canine MCT is surgical excision, which may be suggested whenever possible⁹.

Fine-needle cytology is the basis of MCT diagnostic investigation and is often tried whenever there is the initial doubt. Cytopathology alone can help arriving the correct diagnosis in 92 to 96% of cases. Tissue collection is usually performed for further histopathology and diagnosis. Romanowski technique are effective for staining mast cell granules. In conclusive cases, where mast cell granules staining does not occur, the use of special stains, such as Giemsa and toluidine blue, is recommended¹⁰.

In conclusion, canine MCTs are one of the common neoplasms which can present a high degree of malignancy. These tumour can be accompanied by fever, cellulitis, myositis and diffuse pruritus. Diagnosis can be done based on clinical behaviour, cytology and histopathological findings, however special staining and immunohistochemistry can be done to further access the biological behaviour of the neoplasm. Ultrasonography can be helpful in determining the spread and thus staging of the tumour.

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