Renal Tumors Pathology — RCC and Wilms

RCC Molecular Pathways

• VHL (von Hippel-Lindau) pathway dysfunction — Most common in clear cell RCC (70-80%); loss of VHL protein leads to accumulation of HIF-1α and HIF-2α (hypoxia-inducible factors), which drive transcription of angiogenic factors (VEGF, PDGF), glucose metabolism genes (GLUT1), and other oncogenic pathways even under normoxic conditions. This explains the characteristic hypervascularization and lipid accumulation (clear cell appearance). VHL inactivation occurs through mutation (40%), hypermethylation (20%), or chromosomal loss (50%) on chromosome 3p.

• mTOR pathway activation — Constitutive signaling through mTOR complex 1 (mTORC1) promotes cell proliferation and survival independent of growth factor signals. Occurs through PTEN loss, TSC1/2 mutations, or direct mTOR activating mutations. Explains clinical efficacy of mTOR inhibitors (temsirolimus, everolimus).

• Chromatin remodeling gene mutations — PBRM1 (35-40% of ccRCC), BAP1 (10-15%), and SETD2 (15-20%) are among the most frequently mutated genes in clear cell RCC. These genes regulate histone acetylation and methylation, affecting gene expression. PBRM1 and BAP1 mutations are associated with aggressive behavior and worse prognosis. SETD2 mutations correlate with higher Fuhrman grade and metastatic potential.

• Tyrosine kinase receptor signaling — Activating mutations in MET (papillary RCC type 1), FLCN (Birt-Hogg-Dubé syndrome, chromophobe/oncocytoma), and FH (fumarate hydratase, papillary type 2) drive ligand-independent proliferation. These pathways are targetable by multi-kinase inhibitors (sunitinib, sorafenib, cabozantinib).

Wilms Tumor Molecular Events

• WT1 inactivation — Occurs in 10-15% of sporadic Wilms tumors but in >90% of WAGR syndrome cases (Wilms, Aniridia, GU anomalies, Retardation). WT1 is a zinc-finger transcription factor functioning as a tumor suppressor. Loss permits uncontrolled proliferation of embryonal nephrogenic cells that normally differentiate under WT1 influence. Located on chromosome 11p13.

• WTX loss (CTNNB1 pathway dysregulation) — WTX (Wilms tumor X-linked gene, located at Xq11) mutations occur in 30% of Wilms tumors. WTX regulates β-catenin degradation; loss leads to Wnt/β-catenin pathway hyperactivation and enhanced proliferation and stemness of embryonal kidney cells.

• 11p15 imprinting abnormalities — Loss of imprinting (LOI) or loss of heterozygosity (LOH) at 11p15 (containing IGF2 and H19) occurs in 50% of Wilms tumors (Beckwith-Wiedemann syndrome). Results in increased IGF2 (growth factor) expression and loss of H19 (growth suppressor), driving proliferation of metanephric mesenchyme.

• Triphasic histology reflects developmental abnormality — The characteristic combination of blastemal, epithelial, and stromal components reflects the tumor's derivation from totipotent embryonal renal precursors that can differentiate into multiple tissue types (mesenchymal, epithelial, smooth muscle). This multipotentiality reflects failure of normal differentiation signals.

RCC Risk Factors

• Tobacco use — 20-30% attributable risk; dose-dependent relationship; smoking causes oxidative DNA damage and carcinogen-induced mutations (nitrosamines)
• Obesity and hypertension — Each independently increases risk 1.5-2 fold; may relate to chronic hypoxia, adipokine dysregulation, and endothelial dysfunction
• Chronic kidney disease and dialysis — 50-fold increased risk in ESRD patients; acquired cystic kidney disease provides precursor lesions; uremic toxins and oxidative stress implicated
• Genetic syndromes — Von Hippel-Lindau disease (25% develop RCC, often multifocal/bilateral), Hereditary papillary RCC, Birt-Hogg-Dubé syndrome (FLCN mutations, chromophobe/hybrid tumors), Hereditary leiomyomatosis and RCC (FH mutations, aggressive papillary type 2)
• Prior renal infarction or trauma — Hypoxia and chronic inflammation increase transformation risk
• Environmental exposures — Occupational cadmium, asbestos, organic solvents
• Male predominance — 1.5-2:1 male-to-female ratio

Wilms Tumor Risk Factors

• Aniridia (WAGR syndrome) — WT1 mutations; 30-50% develop bilateral Wilms
• Beckwith-Wiedemann syndrome — 11p15 imprinting defects; 10% develop Wilms (often before age 5)
• Denys-Drash syndrome — WT1 mutations causing XY sex reversal and progressive glomerulonephritis; 90% develop Wilms by age 10
• Sporadic aniridia — 15-20% risk of Wilms development
• Cryptorchidism and hypospadias — Suggest gonadal dysgenesis and increased Wilms risk (1-2%)
• Hemihypertrophy and organomegaly — Associated with Beckwith-Wiedemann; 5-fold increased Wilms risk
• Maternal diabetes — Modestly increased risk (1.5-2 fold)
• Male sex — Slight male predominance (1.1:1)

RCC Clinical Features

• Classic triad (20-30% of cases) — Hematuria (60%), flank pain (40%), palpable flank mass (30%); occurs only when tumor has achieved substantial size and invaded collecting system or retroperitoneal structures
• Hematuria mechanisms — Tumor invasion of collecting system causes mucosal erosion and hemorrhage; often gross and painless; can present with clots causing obstruction and renal colic
• Flank pain — Results from tumor growth causing capsular distention, renal infarction from vascular compression, or hemorrhage into tumor
• Constitutional symptoms — Fever, weight loss, night sweats (10-15% of cases); mediated by IL-6 and TNF production by tumor and infiltrating macrophages
• Hypertension (20-40%) — Mechanisms include renin production by tumor cells, renal artery compression, or loss of renal parenchyma with activation of renin-angiotensin system
• Paraneoplastic syndromes — Polycythemia (EPO production by tumor or peritumoral macrophages, 3-12%), hypercalcemia (PTHrP production, 5%), hypertrophic osteoarthropathy, Stauffer syndrome (hepatic dysfunction without metastases, mediated by IL-6)
• Varicocele — Right-sided varicocele suggests renal vein or IVC thrombosis from direct tumor invasion (10-15% of cases); left-sided varicocele is more commonly idiopathic
• Syncope or acute dyspnea — May indicate IVC thrombosis or pulmonary embolism
• Late presentation — 25-30% present with metastatic disease; lungs (75%), bones (20%), liver (15%)

Wilms Tumor Clinical Features

• Abdominal mass (80-90%) — Usually asymptomatic, discovered on routine physical exam or by parents during bathing; typically unilateral but 5-10% bilateral (either synchronous or metachronous)
• Abdominal distention and mass effect — Tumor growth causes progressive abdominal swelling; may cause early satiety or abdominal pain
• Hematuria (20-30%) — Less frequent than in RCC; indicates invasion into collecting system
• Hypertension (50-60%) — Often severe; mechanisms include renin production, renal artery compression, or mass effect elevating intra-abdominal pressure
• Constitutional symptoms — Fever, malaise, weight loss (10-15%)
• Vaginal bleeding — May occur with papillary histology or extension into bladder/urethra
• Presentation with metastatic disease — 10-15% present with pulmonary or hepatic metastases (lung most common, 80%)
• Associated congenital anomalies — Aniridia (1%), hemihypertrophy (2%), cryptorchidism (3%), hypospadias, cardiac malformations (5%)

RCC — Imaging & Gross Pathology

• CT with IV contrast (gold standard) — Shows heterogeneously enhancing renal mass with rapid arterial phase enhancement (due to hypervascularization); Bosniak classification grades cyst complexity and malignancy risk. Assesses tumor thrombus in renal vein/IVC (critical for surgical planning), local invasion, regional lymph node involvement, and distant metastases.

• MRI — Superior for thrombus evaluation (IVC extension); useful in patients with contrast allergy; T1 hyperintensity reflects lipid and hemorrhage content.

• Renal ultrasound — Often incidental finding; shows heterogeneous echogenic mass, useful for Doppler assessment of venous thrombosis and cost-effective surveillance.

• Gross pathology appearance — Depends on histological subtype:
• Clear cell RCC: Golden-yellow, homogeneous appearance due to abundant intracytoplasmic lipid; may show areas of hemorrhage (dark red), necrosis (tan), or cystic change; usually well-demarcated but lacks true capsule
• Papillary RCC: Tan-gray, smaller tumors with prominent fibrovascular architecture; usually well-circumscribed
• Chromophobe RCC: Tan, homogeneous appearance with characteristic "renal oncocytoma-like" features but larger and more infiltrative than true oncocytoma
• Collecting duct carcinoma: White, infiltrative mass with poorly defined borders; usually arises near renal hilum

RCC — Histopathology

• Clear cell RCC (70% of RCC) — Cells with abundant clear cytoplasm (lipid and glycogen) arranged in nested or solid sheets, separated by delicate fibrovascular septa; nuclei are low to intermediate grade; chromatin appears "salt-and-pepper" (fine dispersion). Fuhrman nuclear grading (Grade 1-4) based on nuclear size, irregularity, and nucleolar prominence is critical prognostic factor. Variants include granular cell (abundant mitochondria causing eosinophilic cytoplasm, suggests poor prognosis), sarcomatoid (spindle cell morphology indicating high-grade transformation, highly aggressive), and rhabdoid (primitive appearing with aggressive behavior).

• Papillary RCC (10-15% of RCC) — Cuboidal to columnar cells with pale or eosinophilic cytoplasm arranged on fibrovascular cores creating papillary architecture; foamy macrophages present in cores; often associated with psammoma bodies (laminated calcifications). Type 1 (low-grade, MET-driven) has smaller cells and better prognosis; Type 2 (high-grade, FH-driven) has larger cells with prominent nucleoli and worse outcomes.

• Chromophobe RCC (5% of RCC) — Cells with pale, flocculent ("plant cell-like") cytoplasm due to numerous mitochondria; characteristic "binucleate cells" and "wrinkled tissue paper" or "chicken-wire" nuclear membranes (best seen on electron microscopy); lacks lipid unlike clear cell. Generally lower grade and better prognosis than clear cell RCC. Oncocytoma (benign tumor) shows identical histology but smaller size, smaller nuclei, and lacks atypia.

• Collecting duct carcinoma (<1% of RCC) — High-grade adenocarcinoma with irregular glandular formations and prominent desmoplasia; arises from medullary collecting ducts; extremely aggressive with poor prognosis.

• Sarcomatoid RCC (variable, seen in 5-10% of RCC) — Spindle cell morphology resembling sarcoma with areas of conventional RCC histology; represents high-grade dedifferentiation; highly aggressive regardless of underlying histological type.

RCC — Immunohistochemistry

• CD10+ (membranous and cytoplasmic) — Positive in clear cell and papillary RCC; helps distinguish from other renal tumors
• RCC antigen (positive) — Present in clear cell and papillary RCC
• PAX8+ (nuclear) — Positive in papillary RCC and chromophobe RCC; helps distinguish papillary from other subtypes
• CK7+ (variable) — Often positive in papillary and chromophobe RCC; negative in clear cell
• CAIX (CA IX, positive) — Hypoxia-inducible carbonic anhydrase IX; present in clear cell RCC due to HIF accumulation; useful diagnostic marker
• Chromophobe: EMA+, claudin+, cytokeratin 7+

RCC — Staging (TNM)

• Stage I: T1 (≤4 cm) — confined to kidney
• Stage II: T2 (4-7 cm or >7 cm) — confined to kidney
• Stage III: T1-2N1M0 (regional LN metastases) or T3 (adrenal invasion, renal vein/IVC invasion)
• Stage IV: T4 (Gerota's fascia invasion) or M1 (distant metastases)

RCC — Prognostic Markers

• Fuhrman nuclear grade — Grade 1-2 has 80-90% 5-year survival; Grade 3-4 has 20-50%; most important histological prognostic factor
• Sarcomatoid/rhabdoid features — Indicate dedifferentiation and aggressive behavior; poor prognosis
• Tumor necrosis — Indicates rapid growth and hypoxia; independent adverse prognostic factor
• Hilar/perinephric fat invasion and vascular invasion — Both adversely affect prognosis
• ECOG performance status — Strong predictor of survival in metastatic disease
• Molecular features: BAP1 and SETD2 mutations associated with worse outcomes; PBRM1 mutations suggest intermediate prognosis

Wilms Tumor — Imaging & Gross Pathology

• Abdominal ultrasound — First-line imaging in children; shows heterogeneous, predominantly solid intrarenal mass with variable echogenicity (cystic areas common), usually unilateral
• CT chest/abdomen/pelvis — Assesses contralateral kidney (4-7% bilateral), local tumor extent, regional lymph nodes, and pulmonary/hepatic metastases
• MRI —

Localized RCC (AUA Renal Mass/Localized Renal Cancer guideline; NCCN Kidney Cancer)

• Partial nephrectomy: preferred for cT1 masses — nephron-sparing surgery preserves GFR and gives equivalent cancer control.
• Radical nephrectomy: for large, central, or locally advanced tumors, or when partial resection is not technically feasible.
• Thermal ablation or active surveillance: reasonable for small (<3 cm) masses in elderly or comorbid patients; percutaneous biopsy is used when the result would change management.
• Adjuvant pembrolizumab (anti–PD-1) is an option after resection of high-risk clear cell disease per NCCN.

Advanced/metastatic RCC (NCCN)

• First line — immune checkpoint inhibitor–based doublets: ipilimumab plus nivolumab, or an anti–PD-1 combined with a VEGF tyrosine kinase inhibitor (e.g., pembrolizumab plus axitinib).
• Escalation/second line — VEGF-directed TKIs: cabozantinib, or lenvatinib plus the mTOR inhibitor everolimus.
• HIF-2α inhibitor (belzutifan): mechanistically elegant in VHL-associated disease, where HIF-2α accumulation drives the tumor.
• Cytoreductive nephrectomy and metastasectomy in selected patients.
• Contraindicated/ineffective: conventional cytotoxic chemotherapy and radiotherapy — RCC is classically chemoresistant and radioresistant (radiation is palliative only, e.g., for bone or brain metastases).

Wilms tumor (Children's Oncology Group approach used in the US)

• Upfront radical nephrectomy with regional lymph node sampling for resectable unilateral disease, followed by risk-adapted chemotherapy — vincristine plus dactinomycin, adding doxorubicin for higher stage or unfavorable histology; flank/whole-abdomen radiation for stage III and above or anaplastic histology.
• Bilateral or syndromic disease: preoperative chemotherapy then nephron-sparing surgery to preserve renal mass (the SIOP European model gives preoperative chemotherapy to all).
• Avoid: intraoperative spillage or preoperative biopsy of a resectable tumor — rupture upstages the disease and mandates abdominal radiation.

Disease-related — RCC

• Tumor thrombus in renal vein/IVC: direct intravascular growth; signalled by a non-reducing varicocele, lower-extremity edema, or new proteinuria. Extension to the right atrium with embolization causes pulmonary embolism — an emergency.
• Hemorrhage into tumor / Wunderlich syndrome: rupture of a hypervascular mass into the perinephric space; sudden flank pain, hypotension, falling hematocrit — emergency requiring embolization or surgery.
• Hypercalcemia of malignancy: PTHrP secretion or osteolytic bone metastases; confusion, polyuria, short QT — a hypercalcemic crisis is an emergency.
• Pathologic fracture and spinal cord compression: RCC bone metastases are lytic and hypervascular; back pain with neurologic deficit demands emergent MRI and glucocorticoids.
• Polycythemia and hyperviscosity: ectopic erythropoietin.
• Stauffer syndrome: IL-6–driven cholestatic liver dysfunction without hepatic metastases; reverses after nephrectomy.

Disease-related — Wilms

• Tumor rupture (spontaneous or intraoperative): peritoneal seeding, upstaging, and need for abdominal radiation — an intraoperative emergency.
• Severe renin-mediated hypertension and, less often, acquired von Willebrand disease causing bleeding.
• Extension into the renal vein/IVC with the same embolic risk as in adults.

Treatment-related

• Post-nephrectomy CKD: loss of nephron mass with hyperfiltration injury; rising creatinine and proteinuria — the rationale for nephron-sparing surgery.
• VEGF TKIs: hypertension, proteinuria, hand–foot skin reaction, hypothyroidism, impaired wound healing, arterial thrombotic events; hypertensive emergency and GI perforation are the emergencies.
• Checkpoint inhibitors: immune-related colitis, hepatitis, thyroiditis, and hypophysitis; myocarditis (rising troponin, conduction block) is rare but frequently fatal — an emergency treated with high-dose glucocorticoids.
• Doxorubicin: cumulative dose-dependent cardiomyopathy — falling ejection fraction on surveillance echocardiography.
• Dactinomycin: hepatic sinusoidal obstruction syndrome (tender hepatomegaly, weight gain, hyperbilirubinemia).
• Flank/abdominal radiation in children: scoliosis, growth impairment, radiation nephritis, and second malignancies.

• Golden-yellow cut surface with clear cells and delicate vasculature = clear cell RCC; the lipid and glycogen dissolve during processing, producing the clear cytoplasm. Link it to VHL loss on chromosome 3p and HIF-driven VEGF — this is the single association examiners test most.
• **RCC is the *internist's tumor***: paraneoplastic polycythemia (EPO), hypercalcemia (PTHrP), and Stauffer syndrome (IL-6 cholestasis with no liver metastases). A patient with unexplained erythrocytosis and a renal mass is not polycythemia vera — check the erythropoietin level (it is high, not suppressed).
• Best next step for an enhancing solid renal mass: contrast-enhanced CT of the abdomen with dedicated evaluation of the renal vein and IVC, then surgical planning — not biopsy in most resectable cases.
• RCC is chemoresistant and radioresistant; the common distractor is offering cisplatin-based chemotherapy or definitive radiation instead of nephrectomy and checkpoint inhibitor–based systemic therapy.
• Wilms tumor: triphasic blastemal, epithelial, and stromal histology in a child under 5 with a smooth flank mass that does not cross the midline. The distractor is neuroblastoma — which crosses the midline, calcifies, elevates urinary VMA/HVA, and shows Homer-Wright rosettes and opsoclonus-myoclonus.
• Anaplasia (marked nuclear enlargement, hyperchromasia, atypical mitoses) defines unfavorable histology Wilms, correlates with TP53 mutation, and drives intensified therapy — it is the histologic finding that changes prognosis, not tumor size.
• Syndrome mapping: WT1 at 11p13 → WAGR and Denys–Drash (nephrotic syndrome plus 46,XY disorder of sex development); 11p15 IGF2 loss of imprinting → Beckwith–Wiedemann (macroglossia, omphalocele, hemihypertrophy). Children with these predisposition syndromes undergo serial abdominal ultrasonography through early childhood.
• Benign mimics: oncocytoma (mahogany-brown with a central stellate scar) and angiomyolipoma (macroscopic fat on CT, HMB-45 positive, tuberous sclerosis) — fat within a renal mass argues strongly against RCC.