An older woman with a renal mass

Jun Ma, M.D.

Specimen Type:

Kidney

History:

A 61-year-old woman with a left renal mass underwent nephrectomy. Grossly, a centrally placed well demarcated tumorous mass that measures 6cm in its largest diameter. It shows somewhat gelatinous appearance. Representative cross-sections were submitted.

Pathologic Features:

The tumor cells are arranged to form nests, tubules and papillary structures (Fig 1-4). The acini are very small, crowding, separated by acellular stroma. The cytoplasm is pink or clear, nuclear chromatin is delicate, nucleoli are inconspicuous, and mitotic figures are absent. The tumor cells are positive for CKAE1/AE3 and vimentin.

Differential Diagnosis:

  • Epithelial-predominant Wilms tumor (WT).
  • The solid variant of papillary renal cell carcinoma (PRCC).
  • Mixed epithelial and stromal tumor.
  • Metanephric adenoma / Metanephric adenofibroma of the Kidney

Diagnosis:

Metanephric adenoma / Metanephric adenofibroma of the Kidney

Key Features:

  • Older patients, female dominate.
  • Well-circumscribed renal mass.
  • Tumor cells arranged in nests, tubules or form papillae.
  • Closely packed small acini separated by acellular stroma.
  • Clear to pink cytoplasm, delicate nuclear chromatin, inconspicuous nucleoli, absent or rare mitotic figures.
  • Tumor cells positive for CKAE1/AE3 and vimentin.

Comment:

Originally described in the French literature in 1980 as nephrogenic nephroma, and subsequently named metanephric adenoma. Currently, more than 100 patients with metanephric adenoma have been reported. Its name reflects its morphologic resemblance to the cytologic and architectural features of early metanephric tubular differentiation and to the metanephric hamartomatous element of nephroblastomatosis. metanephric adenofibroma, it was recognized that a separate subset of renal neoplasms consisted entirely of stromal elements identical to the stromal component of metanephric adenofibroma.

The majority of patients are female, with a female: male ratio of about 2:1. The reported age range is 15 months to 83 years. In at least half of cases, the tumor produces no symptoms and is discovered incidentally.

Metanephric adenoma ranges in size from 0.3 to 20 cm in greatest dimension, a mean of 5.5 cm. It is typically unilateral and rarely multifocal. The majority are either unencapsulated or have only a limited and discontinuous pseudocapsule.

Microscopically, tumor cells are arranged to form nests and tubules, papillary structures are noted. The acini are very small separated by acellular stroma, consisting only of edema fluid or a smoothly hyalinized matrix. Tumor cells possess little cytoplasm, which is usually pink or clear. Nuclei are slightly bigger than lymphocytes, irregularly rounded or ovoid. Nuclear chromatin is delicate, nucleoli are absent or inconspicuous, and mitotic figures are rare or entirely absent.

The differential diagnosis of metanephric adenoma is essentially limited to epithelial-predominant Wilms tumor (WT) and the solid variant of papillary renal cell carcinoma (PRCC). Staining profiles for most keratin markers and vimentin are inconsistent. Metanephrivc adenoma is typically immunopositive for WT1 and CD57 and immunonegative for racemase and CK7, an immunoprofile that is the exact opposite of that of PRCC. DNA content analysis by flow cytometry yields diploid histograms.

Most investigators regard metanephric adenoma as a benign neoplasm. In one case, a nephrectomy specimen that harbored metanephric adenoma also had demonstrable regional lymph node metastases. It has been suggested that in this case, the underlying neoplasm was a Wilms tumor that metastasized and subsequently matured into a metanephric adenoma. Other cases of metanephric adenomas with putative malignant behavior have been less compelling.

References:

  1. Bostwick DG, Cheng L., UROLOGIC SURGICAL PATHOLOGY 2ND EDITION, MOSBY, 2008, p113-117.