Definition/General

Introduction:
-Renal oncocytoma is a benign neoplasm composed of oncocytic cells
-It represents 3-7% of all renal neoplasms
-It arises from the intercalated cells of the collecting duct system
-FNAC shows characteristic oncocytic features that aid in diagnosis.
Origin:
-Originates from the intercalated cells of the cortical collecting ducts
-The tumor cells demonstrate abundant eosinophilic cytoplasm packed with mitochondria
-The cells show uniform appearance without significant pleomorphism
-Benign cytological features are characteristic.
Classification:
-Classified as benign renal neoplasm according to WHO 2016
-Classic oncocytoma (typical morphology)
-Oncocytoma with chromophobe features (hybrid tumor)
-FNAC may not always distinguish from chromophobe RCC
-No grading system required due to benign nature.
Epidemiology:
-Peak incidence in 6th-7th decades
-Male to female ratio is 2:1
-Associated with Birt-Hogg-Dubé syndrome
-Tuberous sclerosis complex also associated
-Multiple oncocytomas possible
-Generally excellent prognosis with surgical resection.

Clinical Features

Presentation:
-Incidental finding on imaging (majority of cases)
-Abdominal mass (if large)
-Flank pain (uncommon)
-Hematuria (rare)
-Usually completely asymptomatic
-No paraneoplastic syndromes
-May be multiple in syndrome patients.
Symptoms:
-Most patients are completely asymptomatic
-Vague abdominal discomfort if tumor is large
-Hematuria very rare
-No constitutional symptoms
-No fever or weight loss
-Symptoms usually only related to mass effect.
Risk Factors:
-Birt-Hogg-Dubé syndrome (multiple renal oncocytomas)
-Tuberous sclerosis complex
-Renal oncocytosis (diffuse oncocytic change)
-Family history in syndrome patients
-No strong environmental risk factors
-Male predominance.
Screening:
-Syndrome patients require regular renal imaging
-Annual screening for BHD syndrome
-Genetic counseling for familial cases
-MRI preferred for young patients
-Incidental finding in most sporadic cases.

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Gross Description

Appearance:
-Well-circumscribed round mass with mahogany-brown cut surface
-Central stellate scar characteristic (60-70% cases)
-Homogeneous appearance
-Size typically small to medium (2-8 cm)
-Well-defined capsule usually present.
Characteristics:
-Mahogany-brown to tan cut surface with homogeneous appearance
-Central stellate fibrous scar pathognomonic feature
-No significant necrosis or hemorrhage
-Solid, fleshy consistency
-Calcifications rare
-Cystic change uncommon.
Size Location:
-Variable size (1-12 cm, average 4-6 cm)
-Can occur anywhere in kidney
-Unilateral presentation typical (except in syndromes)
-Cortical location preferred
-Multiple tumors possible in BHD syndrome
-Bilateral involvement in syndrome patients.

Microscopic Description

Immunohistochemistry

Positive Markers:
-Mitochondrial markers (anti-mitochondrial antibody)
-PAX8 (renal origin)
-E-cadherin (positive)
-EMA (positive)
-CK7 (variable, usually negative or focal)
-S-100 protein (negative, helps exclude chromophobe).
Negative Markers:
-CD117 (Kit) (negative, distinguishes from chromophobe RCC)
-Hale colloidal iron (negative)
-Vimentin (negative)
-CD10 (negative)
-AMACR (P504S) (negative)
-Melanoma markers (negative).
Diagnostic Utility:
-CD117 negativity distinguishes from chromophobe RCC
-Mitochondrial markers highlight abundant mitochondria
-Hale colloidal iron negativity helps exclude chromophobe RCC
-CK7 negativity supports oncocytoma (but not absolute)
-Morphology remains most important.

Molecular/Genetic

Genetic Mutations:
-Normal diploid karyotype characteristic
-Mitochondrial DNA mutations and deletions
-FLCN gene mutations (Birt-Hogg-Dubé syndrome)
-TSC1/TSC2 mutations (tuberous sclerosis)
-No recurrent chromosomal aberrations like other RCC subtypes.
Prognostic Significance:
-Excellent prognosis with complete surgical resection
-No metastatic potential
-No malignant transformation reported
-Recurrence rare with complete excision
-Syndrome patients may develop multiple tumors over time.

Differential Diagnosis

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

FNAC from [kidney mass/renal lesion], [location], performed under [guidance method]

Specimen Adequacy

[Adequate/Inadequate] for cytological interpretation

Cytological Findings

Cellular smears showing [cellularity] with [uniform oncocytic cells]. Cells show [abundant granular eosinophilic cytoplasm] and [uniform round nuclei]. [Smooth cell borders] noted

Key Morphological Features

[Uniform oncocytic cells/Benign nuclear features/Smooth cell borders/Abundant eosinophilic cytoplasm] observed

Background

Background shows [clean background/minimal inflammatory infiltrate]

Cytological Diagnosis

[Benign/Favor benign] - Features consistent with oncocytoma

Differential Diagnosis

Main differential: [Chromophobe RCC/Papillary RCC Type 2/Other oncocytic tumors]

Limitations

FNAC has limitations in definitively distinguishing oncocytoma from chromophobe RCC

Recommendations

[Histopathological examination/Core biopsy/Immunohistochemistry panel] recommended for definitive diagnosis

Note

Final diagnosis requires histological examination with immunohistochemistry and clinical-radiological correlation