Histopathologic, phenotypic, and molecular criteria to discriminate low‐grade intestinal T‐cell lymphoma in cats from lymphoplasmacytic enteritis

Background Differentiation of low‐grade intestinal T‐cell lymphoma (LGITL) from lymphoplasmacytic enteritis (LPE) in cats is a diagnostic challenge for pathologists. Objective Characterize histologic, immunohistochemical, and molecular features of LGITL and LPE. Animals Forty‐four client‐owned cats, 22 diagnosed with LGITL and 22 with LPE. Methods Prospective, cohort study. Clinical suspicion of LGITL or LPE was based on persistent gastrointestinal signs, unresponsive to empirical treatments. All cats underwent a standardized diagnostic evaluation, including biopsy (preferentially full‐thickness), and were diagnosed with LGITL or LPE after review of clinical, laboratory, sonographic, histologic, immunohistochemical, and clonality results. Results A monomorphic lymphocytic population (22/22, 100%) and in‐depth mucosal infiltration (15/22, 68%) were hallmarks of LGITL. Epithelial patterns (nests and plaques) were significantly more frequent in LGITL (11/22, 50%) than in LPE (1/22, 5%) cases (P = .001). A CD3+ lymphocytic apical‐to‐basal gradient was observed in 9/22 (41%) of LGITL vs 1/22 (5%) of LPE cases (P = .004). Most LPE cases (17/18, 94%) featured marked fibrosis in the superficial part of the lamina propria. The Ki‐67 20%‐ and 30%‐thresholds discriminated between LGITL and LPE within both the epithelium (specificity >95%) and lamina propria (specificity >95%), respectively. All LGITL cases were CD3+ pSTAT3− and pSTAT5+. T‐cell receptor gamma chain gene rearrangements indicated monoclonality in 86% of LGITL cases. Surprisingly, 70% of LPE cases featured monoclonality (40%) or monoclonality on a polyclonal background (30%). Conclusions and Clinical Importance We identified new histologic, immunohistochemical, and clonality criteria to distinguish LGITL from LPE.