Definition/General
Introduction:
Renal oncocytoma is a benign epithelial tumor of the kidney
Represents 3-7% of renal neoplasms
Composed of oncocytes with abundant mitochondria
Shows excellent prognosis.
Origin:
Arises from intercalated cells of collecting duct
Shows oncocytic differentiation
Mitochondrial proliferation
Associated with Birt-Hogg-Dubé syndrome.
Classification:
WHO 2022: Benign oncocytic neoplasm
Classic oncocytoma
Oncocytoma with hybrid features
Part of oncocytic tumor family.
Epidemiology:
Peak incidence 6th-7th decades
Male predominance (2:1)
Usually solitary
Multifocal/bilateral in BHD syndrome
Incidental finding common.
Clinical Features
Presentation:
Asymptomatic (70%)
Incidental finding
Flank pain (large tumors)
Hematuria (rare)
Excellent prognosis.
Symptoms:
Usually asymptomatic
Abdominal/flank discomfort
Palpable mass (large tumors)
Hematuria uncommon
No constitutional symptoms.
Risk Factors:
BHD syndrome (FLCN mutations)
Multiple renal cysts
Pulmonary cysts
Skin fibrofolliculomas
Family history.
Screening:
CT/MRI imaging
Central scar pathognomonic
Spoke-wheel enhancement
Genetic testing (BHD)
Family screening.
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Gross Description
Appearance:
Well-circumscribed
Tan-brown color
Homogeneous cut surface
Central stellate scar (70%)
Soft consistency
Lobulated surface.
Characteristics:
Smooth capsule
Mahogany brown color
Spoke-wheel pattern
Central fibrous scar
No necrosis
Uniform appearance.
Size Location:
Variable size (1-20 cm)
Any location in kidney
Cortical based
Well-demarcated
Multiple/bilateral (BHD)
Exophytic growth.
Multifocality:
Usually solitary
Multifocal in 5-13%
Bilateral in BHD syndrome
Hybrid tumors with chromophobe RCC
Oncocytosis (diffuse oncocytic change).
Microscopic Description
Histological Features:
Nests of oncocytes
Abundant eosinophilic cytoplasm
Round to oval nuclei
Regular nuclear contours
Edematous stroma
Hyalinized areas.
Cellular Characteristics:
Large polygonal cells
Abundant granular cytoplasm
Uniform round nuclei
Single prominent nucleolus
Well-defined cell borders
No pleomorphism.
Architectural Patterns:
Nested/alveolar pattern
Solid sheets
Occasional tubules
Central hyalinized scar
Edematous stroma
Prominent vasculature.
Grading Criteria:
Benign tumor
No grading applicable
Assessment: Cellular uniformity
Nuclear regularity
Absence of mitoses
No necrosis.
Immunohistochemistry
Positive Markers:
Anti-mitochondrial antibody - positive
S-100A1 - positive
E-cadherin - positive
EMA - positive
CK8/18 - positive
PAX8 - positive.
Negative Markers:
CK7 - negative (vs chromophobe RCC)
CD117 - negative
Parvalbumin - negative
CK20 - negative
Vimentin - negative.
Diagnostic Utility:
S-100A1 positivity highly specific
CK7 negativity distinguishes from chromophobe RCC
CD117 negative (vs chromophobe)
Mitochondrial antibody highlights cytoplasm.
Molecular Subtypes:
Classic oncocytoma: S-100A1+, CK7-, CD117-
BHD-associated: Same pattern
Hybrid tumors: Mixed immunoprofile
All oncocytomas: Rich mitochondrial content.
Molecular/Genetic
Genetic Mutations:
FLCN mutations (BHD syndrome)
Chromosome 1 rearrangements
Mitochondrial DNA alterations
Simple karyotype
Loss of heterozygosity (chromosome 1).
Molecular Markers:
Mitochondrial proliferation
FLCN/mTOR pathway
Oxidative phosphorylation upregulation
Low proliferation markers
Normal p53.
Prognostic Significance:
Excellent prognosis
No malignant potential
No metastases
Recurrence rare
Surveillance acceptable
Nephron-sparing surgery preferred.
Therapeutic Targets:
Active surveillance
Partial nephrectomy
Ablative techniques
No systemic therapy needed
BHD syndrome management.
Differential Diagnosis
Similar Entities:
Chromophobe RCC
Eosinophilic solid and cystic RCC
Clear cell RCC with eosinophilic cytoplasm
Metastatic adenocarcinoma
Granular cell tumor.
Distinguishing Features:
Oncocytoma: S-100A1+, CK7-, CD117-
Chromophobe RCC: CK7+, CD117+, perinuclear halos
Eosinophilic RCC: Stippled nuclei
Clear cell RCC: CA9+
Electron microscopy: Abundant mitochondria.
Diagnostic Challenges:
Chromophobe RCC (eosinophilic variant)
Hybrid oncocytoma-chromophobe tumor
Limited biopsy material
Crush artifact
Molecular studies helpful.
Rare Variants:
Oncocytoma with papillary features
Hybrid oncocytoma-chromophobe tumor
Oncocytomatosis
Warthin-like oncocytoma.
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Specimen Information
[Partial/radical] nephrectomy with well-circumscribed tumor
Tumor Description
Oncocytoma measuring [X] cm with tan-brown color and [central scar present/absent]
Microscopic Features
Nested arrangement of oncocytes with abundant eosinophilic granular cytoplasm, uniform round nuclei, and no cellular atypia
Central Scar
Central stellate scar: [present/absent]
Immunohistochemistry
S-100A1: Positive, CK7: Negative, CD117: Negative, Anti-mitochondrial: Positive
BHD Syndrome
[Multiple lesions suggest BHD syndrome - genetic counseling recommended / Solitary lesion]
Final Diagnosis
Renal oncocytoma, [X] cm, benign
Management
Benign tumor. Excellent prognosis. No additional treatment required. Consider surveillance if partial nephrectomy.