Case Name: Case 1 - Mast cell tumor Signalment: 7-year old castrated male domestic shorthair cat Presenting Complaint and Patient History: Cutaneous masses have been present on the left hock and right flank for approximately eight months.  The masses were previously determined to be mast cell tumors, and the mass on the right flank was recently excised. Physical Examination: A diffuse area of alopecia and swelling (~2x3 cm) is present over the left calcaneus.  No palpable mass is present at the previous surgery site on the right flank.  The remainder of the exam is within normal limits. Cytology Images: 40X 20X 100X Cytology Description Smears made of needle aspirates from the left popliteal lymph node are extremely cellular, and contain many well-granulated mast cells mixed with a moderate number of lymphocytes.  The lymphocytes are mostly small mature cells, with a few medium and large lymphocytes and occasional plasma cells.  Most of the mast cells are quite heavily granulated, and free mast cell granules are numerous in the background. Opinion:  Mast cell tumor metastasis to the lymph node Histopathology Images: H&E; 40X PAS; 100X   Histopathology Description Histopathology is of tissue from the mass on the left hock. H&E, 40X, 100X:  Neoplastic mast cells efface the tissue architecture.  Individual mast cells have distinct cell borders and abundant, finely granular, blue-gray cytoplasm. Periodic acid-Schiff (PAS), 40X:  Neoplastic mast cells contain intracytoplasmic vacuoles that stain faintly with the Periodic acid-Schiff reagent. Diagnosis:  Mast cell tumor Thoracic radiographs were unremarkable, with no evidence of tumor metastasis. Abdominal ultrasound examination also showed no significant abnormalities. Additional Information: Cytology of a splenic aspirate was normal.  Left hindlimb amputation was performed.