Mast cell tumors (MCTs) are highly invasive and metastatic and are the most frequent round cell tumors in dogs, comprising 16–21% of all cutaneous tumors diagnosed [1, 2], and are the most common skin tumors in dogs [3], representing about 7% to 21% of all dog skin tumors and 11% to 27% of malignant skin tumors in this species [4]. Canine MCTs have variable biologic behaviors, ranging from solitary benign masses that can be cured with surgery alone to systemic and potentially fatal metastatic disease [5], and are always considered potentially malignant, but their true metastatic potential is not entirely known [6].

Round cell tumors consist of discrete cells that are round to oval rather than fusiform. Included in this group are mast cell tumor, histiocytoma, lymphosarcoma (including reticulum cell sarcoma) and transmissible venereal tumor. The differential diagnosis of round cell tumors by histologic examination without concomitant cytologic characterization may, in some instances, depend more on age of animal, growth rate, location of tumor, number of tumors and lymph node involvement rather than histologic criteria. Mastocytosis is a rare group of diseases characterized by abnormal proliferation and infiltration of mast cells within the tissues. The skin is most frequently involved but mast cells also accumulate in the bone marrow, gastrointestinal tract, lymph nodes, spleen, and liver. Mast cell tumors in association with oncocytes thus should be considered supportive evidence of an FNA diagnosis of adenolymphoma. Mastocytosis should be considered in the differential diagnosis of urticaria and solitary mastocytomas may remain active into adolescence, raising concern for systemic progression [7–13].

MCTs are most often graded histologically by the scheme described by Patnaik et al. [14]. Tumours of grade I are circumscribed, mainly dermal in location, and consist of well-differentiated mast cells with prominent metachromatic cytoplasmic granules. Poor cellular differentiation, aggressive growth pattern, moderate to high mitotic activity, mitotic atypia and sparse to absent cytoplasmic granulation are the major features of grade III tumours. Grade II MCTs constitute an intermediary form between the two other grades, but they tend to infiltrate more deeply than grade I neoplasms [14, 15]. MCTs are routinely diagnosed by cytology and histopathology.

Dogs are at risk for cutaneous MCT, which accounts for up to 21% of all skin tumours [3]. The diagnosis of MCT by cytology or histopathology is straight forward in the majority of cases, but forming an accurate prognosis is more challenging [5]. Prognostic factors of significance included grading (cytology and histopathology), staging (regional and distant metastases), breed, tumour localisation and treatment (surgery, radiation and chemotherapy). Cytological examination after fine needle aspiration is useful in establishing the diagnosis but histopathology is needed for grading [18]. Cytology often is helpful in the diagnosis of MCTs because of the characteristic appearance of mast cells with routine staining. As is the case with findings from other species, Wright–Giemsa stain resulted in more intensely stained granules in the neoplastic mast cells. The mechanism of the differences in staining is unclear. Several subtypes of mast cells have been identified in humans and dog based primarily on granule contents and biological function.

This study clearly showed a cellular infiltrate of mast cells, mononuclear cells and eosinophils in the thickened, hyperplastic, and hyperkeratinized epidermis. Mast cells and eosinophils, as opposed to the mononuclear cells of delayed hypersensitivity, predominated, suggesting an immediate hypersensitivity reaction. Once activated, mast cells at the edge of a wound are known to release inflammatory mediators within injured tissue by degranulation [19]. Thus, it would seem that immediate hypersensitivity reactions may be responsible for the development of skin lesions due to tumor in dogs. However, the present results suggest that the intense proliferation of mast cells following tumor occurs predominantly in the skin.

The results of this study suggest that cellular proliferation plays a significant role in the progression of canine MCTs. Although the results of this study confirm the results of previous studies that have shown the prognostic significance of cellular proliferation in canine MCTs [14, 15, 18, 19] cellular proliferation should not be evaluated as a single prognostic factor for canine MCTs but should be evaluated in tandem with additional prognostic indicators. Furthermore, the histologic characteristics of the MCT cells in this affected dog was moderate to abundant cytoplasm, round nuclei with scattered chromatin, fibrous stroma, and eosinophil infiltration. Little necrosis was seen.

Several grading systems have been proposed to classify canine mast cell tumors. The system most commonly used classifies the tumor from grades I to III, depending on how well the mast cells are differentiated, mitotic activity, location within the skin, invasiveness, and the presence of inflammation or necrosis, with grade III being the most aggressive (characterized by the presence of undifferentiated, immature mast cells with a high potential for metastasis) [20, 21]. In the current case, the presence of multiple large nodules, the extension into the dermis, and the poorly differentiated mast cells could be comparable to grade III described for dogs. Recently, a new 2-tier histologic classification for canine cutaneous tumors has been proposed [22]. This grading system is based on the presence of mitotic figures, multinucleated cells, bizarre nuclei, and karyomegaly to predict biological behavior, likely time to metastasis, new tumor development, and survival time. In the present case, although the neoplasia did not show any of these histological parameters, the presence of multiple cutaneous tumors suggests a malignant behavior. Grade III tumours have a very poor prognosis even with treatment, which might discourage owners from extensive surgery. This may explain the discrepancy in surgery performed in MCT cases of different grades. This result agrees with the findings by Murphy et al. [23] that showed dogs with poorly differentiated tumours had a significantly shorter survival time. These results suggest that all multiple cutaneous MCTs should be treated as individual neoplasms and prognosis should be dependent on individual variables such as grade and not related to the number of tumours present, which was shown not to affect survival. Mitotic index and size were not found to be independent predictors of survival.

In conclusion, this study described cytohistopathology features of canine cutaneous MCTs of the skin region. The histological features of these tumours would suggest that most should be classified as high-grade MCTs. A cutaneous MCT may be diagnosed on the basis of observing the histopathological pattern described in present study. Moreover, the use of grading system for a tumor type may be helpful in distinguishing this type of neoplasm from other malignancies with similar morphology. Furthermore, this study demonstrated that in canine cutaneous MCTs, may be used as a sensitive prognostic indicator of post-surgical tumour behaviour; in addition, it is related to other histopathological variables that are widely accepted as having prognostic value.