Metanephric adenofibroma (MAF)

Epidemiology

This is a rare tumour of young patients (range 5 months to 36 years, median 30 months).

Clinical features

Patients commonly present with polycythaemia which resolves on excision of the tumour.

Macroscopic appearances

The tumour is usually solitary with ill-defined margins and located in the renal medulla. It is usually tan-brown and partially cystic.

Histopathology

This is a biphasic stromal-epithelial tumour, with a wide range of variation in the proportions of the two components. The stromal component differs from that of congenital mesoblastic nephroma in showing nodular variation in cellularity, but closely resembles that of metanephric stromal tumour (MST). The stroma commonly shows onion-skin concentric arrangement around epithelial tubules and blood vessels. There is angiodysplasia or intra-tumoral arterioles with epithelioid transformation of medial smooth muscle. Heterogeneous differentiation into glial tissue (in contact with epithelium and forming glial-epithelial complexes), cartilage and fat is common. The epithelium forms small tubules and blunt, short papillae, frequently with psammoma bodies, identical to the purely epithelial metanephric adenoma (MA). Some cases show foci of tubulopapillary carcinoma, others foci of transition to epithelial-predominant Wilm's tumour (usually in younger patients).

There has been a single report of a metanephric adenosarcoma with a malignant spindel cell component².

Immunohistochemistry

Stroma (based on 16 cases):

CD34

positive (9/11 cases), generally patchily, similar to MST

muscle-specific actin

negative

desmin

negative

GFAP

positive in neuroglial nodules (4/4)

S-100

positive in neuroglial nodules (1/1)

Epithelium:

cytokeratin 7

In some cases the MA-like epithelium is focally positive. The foci of tubulopapillary carcinoma are strongly and diffusely positive (cf the diffuse positivity of papillary renal cell carcinoma).

EMA

MA-like epithelium is negative. The foci of tubulopapillary carcinoma are diffusely positive.

Differential diagnosis

MAF is probably related to a number of other metanephric neoplasms:

MAF with Wilm's tumour
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MAF with mitoses
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metanephric stromal tumour

----------------MAF ----------------
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metanephric adenoma

MAF with papillary renal cell carcinoma

Management

Treatment is by excision

Prognosis

The carcinomatous foci may metastasis, but the prognosis is generally good.

References

¹MR Arroyo et al. The spectrum of metanephric adenofibroma and related lesions. J Pathol 2001; 25:433-444.

²Picken, M. M., J. L. Curry, et al. (2001). "Metanephric adenosarcoma in a young adult: morphologic, immunophenotypic, ultrastructural, and fluorescence in situ hybridization analyses: a case report and review of the literature." Am J Surg Pathol 25(11): 1451-7.

CD34	positive (9/11 cases), generally patchily, similar to MST
muscle-specific actin	negative
desmin	negative
GFAP	positive in neuroglial nodules (4/4)
S-100	positive in neuroglial nodules (1/1)

cytokeratin 7	In some cases the MA-like epithelium is focally positive. The foci of tubulopapillary carcinoma are strongly and diffusely positive (cf the diffuse positivity of papillary renal cell carcinoma).
EMA	MA-like epithelium is negative. The foci of tubulopapillary carcinoma are diffusely positive.

	MAF with Wilm's tumour \| \|
	MAF with mitoses \| \|
metanephric stromal tumour	----------------MAF ---------------- \| \|	metanephric adenoma
	MAF with papillary renal cell carcinoma