Renal Cell Neoplasia
Renal cell neoplasia encompasses a biologically diverse spectrum of benign, indolent, and aggressive epithelial tumors of the renal cortex and medulla. While historical schemes relied strictly on cytological appearance and architectural patterns (e.g., clear cell, papillary, chromophobe, oncocytic), contemporary taxonomy incorporates recurrent cytogenetic alterations, signaling pathway dysregulation, and hereditary predisposition syndromes.
Key review & literature clipping:
[[Emerging concepts and recent advances in renal cell neoplasia]] (Virchows Archiv 2026; Trpkov & Agaimy).
1. Taxonomic Shifts & Modern Diagnostic Entities
The Spectrum of Eosinophilic / Oncocytic Renal Neoplasms
Distinguishing benign renal oncocytoma from chromophobe RCC (chRCC) is a classic diagnostic challenge in uropathology. Contemporary investigations establish that "pink cell" tumors represent a broad continuum containing several distinct, mostly indolent entities driven by TSC/mTOR pathway hyperactivation:
Low-Grade Oncocytic Tumor (LOT):
Histology: Solid or nested architecture of uniform low-grade cells with eosinophilic cytoplasm and round, regular nuclei without prominent perinuclear halos or raisins.
Immunophenotype: CK7 diffuse strong positive, CD117 negative.
Genetics: Somatic mutations in TSC1, TSC2, or MTOR.
Behavior: Uniformly indolent.
Eosinophilic Vacuolated Tumor (EVT):
Histology: Distinctive large cytoplasmic vacuoles, prominent round nucleoli, thick-walled entrained intratumoral blood vessels.
Immunophenotype: CD117 positive, CK7 negative or patchy.
Genetics: Mutations in TSC2 or MTOR.
Behavior: Indolent.
Eosinophilic Solid and Cystic (ESC) RCC:
Histology: Solid and micro/macrocystic growth with coarse basophilic cytoplasmic stippling (representing clustered microcalcifications or altered rough endoplasmic reticulum).
Immunophenotype: CK20 positive (often focal), CD117 variable.
Genetics: Biallelic TSC1 or TSC2 loss.
Aggressive MTOR-Mutated Eosinophilic RCC:
Alert: While most TSC/mTOR-mutated tumors are indolent, recent multi-institutional reports (Kwon et al. 2025/2026) document that activating MTOR mutations combined with loss of chromosome 1 (monosomy 1) produce aggressive carcinomas with high metastatic potential.
2. Consolidation of Renal Hemangioblastoma into RCC-FMS
Historically, tumors exhibiting hemangioblastoma-like vascularity and lipidized polygonal cells in the kidney were reported as "renal hemangioblastoma" (RHB).
The Unification: Multi-institutional evidence (Zhao et al., Baranova & Trpkov 2026) demonstrates that pure RHB and hemangioblastoma-like areas in renal cell carcinoma with fibromyomatous stroma (RCC-FMS) share identical clinical, immunophenotypic, and molecular features.
Molecular Driver: Driven by TSC/mTOR pathway alterations (TSC1, TSC2, MTOR mutations), completely distinct from CNS hemangioblastomas (which are driven by VHL mutations in von Hippel-Lindau disease).
Taxonomic Verdict: Trpkov & Agaimy advocate eliminating RHB as an independent entity and classifying these tumors within the RCC-FMS continuum as an indolent morphological variant.
3. FLCN-Driven Renal Neoplasms: Conventional vs. Aggressive Phenotypes
Mutations in the folliculin gene (FLCN) underlie Birt-Hogg-Dubé (BHD) syndrome (associated with pulmonary cysts, spontaneous pneumothorax, and fibrofolliculomas) but also occur somatically:
Conventional Phenotype: Hybrid oncocytic-chromophobe tumors (HOCT) or chromophobe-like neoplasms displaying an intricate mosaic of pale clear cells and eosinophilic cells with low-grade nuclear features and indolent clinical course.
Non-Conventional Aggressive Phenotype: Recent multi-institutional studies (Siegmund et al., Wang et al. 2026) have identified FLCN-mutated carcinomas exhibiting high-grade tubulocystic, tubulopapillary, and solid growth patterns, presenting at advanced clinical stage with distant metastatic dissemination and disease-specific mortality.
Practice Pearl: High-grade unclassified papillary or tubulopapillary renal neoplasms that test negative for FH and MET alterations should be evaluated for somatic and germline FLCN mutations.
4. Papillary Renal Neoplasm with Reverse Polarity (PRNRP)
Categorized under papillary RCC in the 2022 WHO classification, comprehensive multi-platform comparisons (Marletta et al. 2026) establish PRNRP as an independent, autonomous low-grade neoplasm:
Architecture & Cytology: Thin, branching papillae lined by a single non-stratified layer of low-grade eosinophilic cells. Nuclei are uniform, round, and situated apically along the luminal border (reversed nuclear polarity), away from the basal lamina.
Immunophenotypic Profile: GATA3 diffusely and strongly positive, L1CAM positive, CK7 positive; CD10 and AMACR typically negative or weak.
Molecular Hallmark: Recurrent activating KRAS exon 2 mutations (e.g. codon 12); strictly lacks trisomies 7 and 17 (the defining cytogenetic abnormalities of classic papillary RCC).
Prognosis: Exceedingly indolent, with zero documented recurrences or metastases in published series.
5. Diagnostic Biomarkers & Algorithmic Triage
GPR143: An MiT transcription-factor regulated transmembrane protein that serves as a highly sensitive surrogate marker for TFE3/TFEB translocations and FLCN/TSC/mTOR hyperactivation, providing superior sensitivity and cleaner staining than GPNMB.
2SC (S-[2-succino]-cysteine) & SDHB: Essential immunohistochemical screening markers for metabolic nephropathies (FH-deficient and SDH-deficient RCC), which mandate immediate genetic counseling and cascade testing.
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