Canine Mast Cell Tumors

Initial work up of a mast cell tumor (MCT)will involve diagnosis by FNA and staging. The initial staging performed will depend on the index of suspicion that the mass is a grade II or III MCT and clinician’s preference. Cytology is generally diagnostic for MCT. However, an incisional biopsy to determine grade is recommended in some cases. The histological grade may be important to determine in some cases, where the surgical approach may be altered by the grade of the tumor. The World Health Organization (WHO) classification scheme for canine mast cell tumours divides them into four stages according to the clinical presentation . Stage I – One tumour confined to the dermis without regional lymph node involvement (Ia – Without systemic signs, Ib – With systemic signs); Stage II – One tumour confined to the dermis with regional lymph node involvement (IIa – Without systemic signs, IIb – With systemic signs); Stage III – Multiple dermal tumours or one large infiltrating tumour with or without regional lymph node involvement (IIIa – Without systemic signs, IIIb – With systemic signs); and Stage IV – Any tumour with distant metastasis or a recurrence with metastasis (including blood and/or bone marrow involvement). Locoregional staging and evaluation for visceral metastasis (to spleen and liver) are the areas that are most important to evaluate when staging canine patients with MCT. Lymph node staging will involve palpation, FNA for cytology and biopsy for histopathology in some cases. One pit fall with cytological evaluation of the lymph nodes is that it can be difficult to differentiate between neoplastic mast cells and normal mast cells that are at the site due to chemotaxis. Krick et al attempted to define the cytological staging of lymph nodes to create consistency in the literature. In that study they defined: Normal – No mast cells seen Reactive lymphoid hyperplasia Reactive node +/rare individual mast cells Possible metastasis – 2-3 mast cell aggregates of 2-3 cells Probably metastasis >3 foci of mast cell aggregates of 2-3 cells and/or 2-5 aggregates of >3 mast cells Certain metastasis Effacement of lymphoid tissue by mast cells and/or presence of aggregated, poorly differentiated MC with pleomorphism, anisocytosis, anisokaryosis, and/or decreased or variable granulation, and or >5 aggregates of >3 MC Although this definition is helpful and may lead to the ability to predict prognosis in future studies, it still creates a clinical problem of what to do with cases that are determined to have possible or probably metastasis. In Krick’s study, 152 dogs with MCT had lymph node evaluation. 63.8% of dogs were considered stage I and 36.2% were considered stage II. In that study, stage II dogs had a shorter survival time and dogs with grade III MCT were more likely to have stage II disease. In cases with enlarged locoregional lymph nodes with questionable or certain metastasis, removal of the affected lymph node at the time of surgery for histopathology and cytoreduction is recommended. Abdominal ultrasound is also recommended for staging MCT in dogs, with ultrasoundguided FNA of the spleen and liver if they appear abnormal. This can also raise questions of true metastasis versus normal mast cells. Recently Stefanello et al evaluated 52 dogs with MCT that had ultrasound guided aspirates of the liver and spleen. 10/52 (19%) dogs had abnormalities of the spleen or liver on cytology and all 10 of these cases had abnormalities noted on ultrasound.