Sarcomatoid and Unclassified Renal Neoplasms: Histology, Immunology, and Genomics.

Sarcomatoid and unclassified renal cell carcinomas (RCCs) represent diagnostically and clinically challenging entities characterized by aggressive biology and poor prognosis. Sarcomatoid transformation denotes high-grade dedifferentiation that may arise from any RCC subtype and is marked histologically by malignant spindle cells forming fascicles devoid of epithelial architecture. These tumors exhibit pronounced nuclear pleomorphism, necrosis, and frequent mitoses. Immunohistochemistry confirms their epithelial origin through markers such as cytokeratin, epithelial membrane antigen (EMA), and PAX8, while programmed death-ligand 1 (PD-L1) and p53 overexpression reflect underlying molecular alterations and therapeutic relevance. Unclassified RCCs, by contrast, encompass tumors that defy standard classification due to mixed or ambiguous morphology; however, recent molecular advances have redefined many cases as distinct genetic subsets. Genomic profiling reveals recurrent mutations in TP53, BAP1, and CDKN2A in sarcomatoid RCC, and NF2, SETD2, or ALK/NTRK fusions in unclassified RCC, indicating convergent pathways of dedifferentiation, chromatin remodeling, and cell-cycle dysregulation. Clinically, both tumor groups correlate with advanced stage, rapid progression, and resistance to vascular endothelial growth factor (VEGF)-targeted therapies, though immune checkpoint inhibitors show emerging benefit, especially in PD-L1-positive sarcomatoid cases. Integrating histopathologic, immunophenotypic, and molecular features enhances diagnostic accuracy, prognostic stratification, and the identification of actionable targets in these high-risk renal neoplasms.