L1CAM Reliably Distinguishes Low-Grade Oncocytic Tumor from Other Eosinophilic Renal Neoplasms: A Multicenter Immunohistochemical Study with Diagnostic Implications

Introduction: Low-grade oncocytic tumor (LOT) is a recently described renal neoplasm characterized by indolent clinical behavior, a small nested architecture, and distinctive immunophenotypic features. Its distinction from other eosinophilic renal tumors, such as oncocytoma, eosinophilic chromophobe renal cell carcinoma (E-chRCC), and eosinophilic vacuolated tumor (EVT), can be challenging due to overlapping features. The L1 cell adhesion molecule (L1CAM) is being increasingly recognized as a potential diagnostic marker for LOT. Aims: To evaluate the diagnostic performance of L1CAM in distinguishing LOT from morphologically and immunophenotypically similar eosinophilic renal neoplasms. Methods: A total of 54 eosinophilic renal tumors (10 LOTs, 22 oncocytomas, 18 E-chRCCs, and 4 EVTs) were retrospectively collected from five academic institutions and reclassified according to the 2022 WHO criteria. All cases underwent histopathologic review and immunohistochemical analysis for CK7, CD117, GATA3, cathepsin K, and L1CAM. Results: L1CAM showed strong membranous expression in all LOTs (100%) and was negative in oncocytoma, E-chRCC, and EVT, yielding 100% sensitivity and specificity. Traditional markers exhibited overlapping patterns among tumor types. Conclusions: Our findings confirm L1CAM as a highly sensitive and specific marker for LOT, effectively distinguishing it from other eosinophilic renal neoplasms. Incorporating L1CAM into diagnostic panels may enhance accuracy, particularly in challenging cases.