Synchronous Low-Grade Alimentary Lymphoma and Gastrointestinal Mast Cell Tumor with a Collision Pattern in a Cat

Synchronous low-grade alimentary lymphoma and gastrointestinal mast cell tumor have recently been described in cats. This report describes synchronous low-grade alimentary lymphoma and gastrointestinal mast cell tumor involving the jejunum in a cat. A 2.3 kg, 15-year-old, spayed female Chinchilla Persian cat presented with vomiting, diarrhea, anorexia, and lethargy. Ultrasonography and computed tomography demonstrated diffuse intestinal wall thickening and a focal jejunal lesion characterized by marked transmural thickening with loss of wall layering. Exploratory laparotomy and full-thickness intestinal biopsies were conducted. Histopathologic and immunohistochemical examination revealed two morphologically distinct neoplastic populations involving different jejunal wall layers. The two populations were anatomically adjacent but remained distinct without convincing intermingling in the examined histologic sections. These findings supported the interpretation of a lesion most consistent with a collision pattern. The affected intestinal segment was surgically resected, and chlorambucil and prednisolone were administered postoperatively. Clinical signs resolved, imaging abnormalities improved, and no evidence of local gastrointestinal mast cell tumor recurrence or progression of low-grade alimentary lymphoma was detected during the 14-month follow-up period. This case expands the currently recognized morphologic spectrum of synchronous feline intestinal neoplasia and highlights the importance of comprehensive histopathologic assessment and full-thickness intestinal biopsies for accurate diagnosis.