Renal adenoma is a small (<15 mm), low-grade papillary epithelial neoplasm of the kidney. No longer recognized as a distinct entity in the WHO 2022 classification — it is histopathologically indistinguishable from papillary renal cell carcinoma and classified as 'low-grade papillary renal neoplasm.' Autopsy series report 7-22% incidence, increasing with age. Usually discovered incidentally and asymptomatic. On imaging, appears as a small, homogeneous, mildly enhancing cortical mass. Size criterion is controversial — the traditional <15 mm cutoff is not considered a reliable malignancy excluder. Papillary histology + low nuclear grade on biopsy is diagnostic, but definitive differentiation by imaging alone is not possible.
Age Range
40-80
Peak Age
60
Gender
Equal
Prevalence
Uncommon
Renal adenoma originates from proximal tubular epithelium. Shows papillary growth pattern — proliferation of tubular structures in papillary configuration. Histologically shares the same cell morphology as papillary RCC type 1: low nuclear grade (Fuhrman 1-2) cuboidal cells with basophilic or eosinophilic cytoplasm, papillary structures with fibrovascular cores. Trisomy 7 and 17 are seen in both adenoma and papillary RCC — genetic profiles also overlap. Therefore WHO no longer considers size-based distinction reliable. On imaging, small size, homogeneous structure, and mild enhancement are typical because papillary histology has low vascularity — minimal neovascularization causes contrast delay and low enhancement.
The combination of low T2 signal + minimal enhancement on corticomedullary phase is highly characteristic of papillary histology. The combination of these two findings strongly suggests papillary adenoma or papillary RCC and clearly differentiates from clear cell RCC (high T2 + intense enhancement).
Small (<15 mm), homogeneous, isodense or mildly hypodense mass in the renal cortex on non-contrast CT. Calcification is rare. No fat content. Mass borders are smooth and well-defined. May be difficult to distinguish from normal parenchyma on non-contrast CT — especially isodense lesions can be missed.
Report Sentence
Small, homogeneous, isodense mass in the renal cortex on non-contrast CT; contrast-enhanced evaluation is recommended.
Mass appears hypodense compared to normal cortical parenchyma on corticomedullary phase — showing mild or minimal enhancement. No significant enhancement. Homogeneous internal structure. In contrast to the intense enhancement of clear cell RCC, papillary histology does not show significant enhancement due to low vascularity.
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Small mass in the renal cortex appearing hypodense relative to normal parenchyma with minimal enhancement on corticomedullary phase, consistent with papillary histology.
Mass shows low signal intensity on T2-weighted sequences — darker than normal renal parenchyma. This finding is characteristic of papillary histology and differentiates from the high T2 signal of clear cell RCC. Homogeneous internal structure. Signal heterogeneity may be visible in the presence of hemorrhagic components.
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Small homogeneous mass with low signal intensity on T2-weighted MRI in the kidney, consistent with papillary histology.
Mass appears isointense or mildly hyperintense on T1-weighted sequences. Hyperintense T1 signal is associated with intratumoral hemorrhage or hemosiderin accumulation — a common finding in papillary histology. Marked T1 hyperintensity should be interpreted in favor of intratumoral hemorrhage.
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Mass shows mildly hyperintense signal on T1-weighted sequences, consistent with intratumoral hemosiderin/hemorrhage.
Appears as a small, homogeneous, isoechoic or mildly hypoechoic cortical mass on US. Smooth borders. No calcification, necrosis, or cystic component expected in internal structure. May be missed on US due to small size — especially isoechoic lesions may be indistinguishable from normal parenchyma. Low vascularity or avascular pattern on Doppler.
Report Sentence
Small, homogeneous, isoechoic mass in the renal cortex on US with low vascularity on Doppler; further evaluation with CT/MRI is recommended.
Criteria
Small cells with basophilic cytoplasm, thin papillary structures, low nuclear grade (Fuhrman 1). Trisomy 7 and 17 common.
Distinct Features
Histologically identical to papillary RCC type 1 — no distinguishing feature beyond size and grade. Most common subtype.
Criteria
Larger cells with eosinophilic cytoplasm, thick papillary structures, pseudostratified nuclei. Less common.
Distinct Features
Histologically similar to papillary RCC type 2 — type 2 RCC has more aggressive course. Tends to be more heterogeneous on imaging.
Criteria
Multiple small papillary adenomas in the same kidney or bilateral. Common in chronic kidney disease and dialysis patients. May develop on the background of acquired cystic kidney disease.
Distinct Features
Multiple small solid nodules each <15 mm. Risk of RCC development is increased — requires close follow-up. Serial US/MRI follow-up recommended.
Distinguishing Feature
Papillary RCC and renal adenoma cannot be reliably distinguished by imaging. Both show low T2, mild enhancement, T1 hyperintensity. Lesions >15 mm are traditionally considered RCC but size alone is not reliable. Biopsy provides definitive differentiation — adenoma shows low nuclear grade (Fuhrman 1) while RCC may show higher grade.
Distinguishing Feature
Oncocytoma shows homogeneous enhancement and central scar while papillary adenoma shows mild enhancement without central scar. Oncocytoma is generally larger. On MRI, oncocytoma shows intermediate-high T2 signal while papillary adenoma shows low T2 signal. Both lesions are benign.
Distinguishing Feature
Fat-poor AML may show low T2 signal (similar to papillary adenoma). However, AML may show signal dropout on chemical shift MRI (intracellular lipid) and demonstrates more intense enhancement on corticomedullary phase. Fat-poor AML is generally homogeneous and well-defined. Definitive differentiation may require biopsy.
Urgency
surveillanceManagement
surveillanceBiopsy
NeededFollow-up
12-monthSince renal adenoma cannot be distinguished from papillary RCC by imaging, clinical management is determined by size and patient profile. Active surveillance (annual US or MRI) is preferred for lesions <10 mm. Biopsy should be considered for 10-15 mm lesions. Lesions >15 mm are considered papillary RCC and surgical treatment is planned. Since WHO 2022 removed the renal adenoma entity, all papillary lesions are evaluated as low-grade papillary renal neoplasms. Low nuclear grade + papillary histology on biopsy supports active surveillance.
Renal adenoma is considered clinically insignificant and usually requires follow-up. Biopsy is not needed if <1 cm solid cortical lesions do not show growth. According to WHO classification, it is a neoplasm of low malignant potential.