Feline Cutaneous Mast Cell Tumors:

Aggressive Clinical Presentations

Abstract

Background: Mast cell tumors (MCTs) in cats are relatively common and are usually described as tumors with indolent biological behavior and a favorable prognosis. However, a subset of cases may present aggressive evolution, with local recurrence, cutaneous dissemination, lymph node involvement, or visceral organ compromise. Due to the absence of a validated grading system and limited prognostic markers for feline mast cell tumors, predicting their biological behavior remains challenging. This report aimed to describe and discuss aggressive clinical presentations, diagnostic findings, treatments, and outcomes of cats diagnosed with cutaneous mast cell tumors.

Cases: Two male mixed-breed cats diagnosed with cutaneous mast cell tumors at the Veterinary Teaching Hospital (VMTH) of Universidade Federal Fluminense (UFF) between 2018 and 2019 were retrospectively evaluated. Case 1. The 1^st case involved an 8-year-old cat presenting with a non-healing ulcerated nodular lesion on the left ear, associated with regurgitation and pruritus. Cytopathological examination suggested mast cell tumor, and unilateral pinnectomy was performed. Histopathology revealed a poorly differentiated cutaneous mast cell tumor with free surgical margins, moderate anisocytosis and anisokaryosis, and increased mitotic activity. Despite adjuvant chemotherapy with vinblastine associated with corticosteroids and supportive medication, the patient developed severe neutropenia and subsequently multiple new cutaneous nodules consistent with tumor dissemination. Progressive clinical deterioration prevented continuation of chemotherapy, and the patient survived 90 days after surgical excision. Case 2. The 2^nd case involved a 2-year-old cat with an abdominal cutaneous mast cell tumor initially classified as histiocytic and excised with tumor-free margins. Tumor recurrence occurred twice at the same site, with subsequent histopathological reclassification as a poorly differentiated mast cell tumor and high proliferative index on immunohistochemistry, including increased Ki-67 expression and absence of c-Kit labeling. During follow-up, the patient developed a mediastinal mass detected on thoracic radiography and recurrent pleural effusion, with cytological findings suggestive of mast cell involvement. The cat was treated with vinblastine-based chemotherapy followed by lomustine and corticosteroids as palliative therapy, in association with repeated thoracentesis, achieving a survival time of 365 days.

Discussion: Although feline cutaneous mast cell tumors are commonly considered benign, the cases presented demonstrated that aggressive biological behavior may occur even in young animals or in tumors initially classified as histiocytic and excised with free margins. Cutaneous recurrence, suspected metastatic spread, systemic clinical manifestations, and variable response to chemotherapy were observed, reinforcing the marked heterogeneity of this neoplasm in cats. These findings highlighted the limitations of current histopathological classifications and the restricted prognostic value of margin status alone. The cases emphasized the importance of thorough clinical staging, careful histopathological and immunohistochemical evaluation, and long-term follow-up. In addition, they suggested that adjuvant or palliative chemotherapy may contribute to prolonged survival in selected cases, despite the lack of standardized protocols. Further studies are required to improve prognostic stratification and to better define the role of adjuvant therapies in feline cutaneous mast cell tumors.

Keywords: feline, tumors, mastocytoma, neoplasia, MCT.