Metanephric Adenoma / Adenofibroma

Epidemiology and Etiology:

• Children and adults
• 40s and 50s most common
• F > M (2:1)
• (M > F for adenofibroma)

Common sites:

Gross features:

• 3-6 cm mostly
• Multifocal commonly
• Well circumscribed but not encapsulated typically
• Grey to tan to yellow cut surface
• Soft or firm
• Foci of haemorrhage and necrosis common
• Calcification 20%
• Small cysts 10%
• Metanephric adenofibroma:
• Solitary
• Tan
• Partially cystic
• Indistinct borders

Histologic features:

• Sharp junction with kidney usually
• No pseudocapsule usually
• Highly cellular epithelial tumour
• Tightly packed small, uniform, round acini
• Almost solid appearance at low power
• Long branching and angulated tubular structures common
• Papillary structures (50%)
• Tiny cysts with protruding blunt papillae reminiscent of immature glomeruli
• Small, uniform, embryonic-appearing cells
• Embryonal appearance
• Monotonous
• Small, uniform nuclei
• Only a little larger than lymphocyte nuclei
• Absent of inconspicuous nucleoli
• Delicate chromatin
• Mitotic figures absent or rare
• Cytoplasm scant, pale or light pink
• Stroma inconspicuous to loose edematous
• Psammoma bodies common
• Hyalinized scar and focal osseous metaplasia of the stroma (10-20%)
• Metanephric adenofibroma:
• Nodules of epithelium identical to metanephric adenoma embedded in sheets of moderately cellular spindle cells
• Irregular border with kidney typically
• Spindle cell component may entrap renal structures
• Fibroblast-like spindle cell component::
• Eosinophilic cytoplasm, pale
• Nuclei oval or fusiform
• Nucleoli inconspicuous
• Few mitotic figures in a minority of cases
• Hyalinization variable
• Myxoid change variable
• Angiodysplasia, glial, cartilaginous, adipose differentiation occasionally

Immunophenotype:

Marker:	Sensitivity:	Specificity:
Keratins
Vimentin
WT-1 (nuclear)	Common
CD57
CD34 (stroma)	frequently
EMA (neg)	Frequently
CK7 (neg)	Frequently

Molecular features:

Other features:

References:

• Eble JN, Sauter G, Epstein JI, Sesterhenn IA. (eds.) WHO Pathology & Genetics: Tumours of the Urinary System and Male Genital Organs (2004)