Imaging Features and Reliability of Percutaneous Biopsy of Metanephric Adenoma of the Kidney.

Purpose: To describe imaging features of metanephric adenomas, assess the reliability of a diagnosis with image-guided percutaneous renal mass biopsy, and evaluate patient survival outcomes.

Materials and methods: In this IRB-approved, HIPAA-compliant retrospective study, our institution's radiology report database was searched for the term "metanephric adenoma" from 2010 to 2020. Patient information, imaging mass characteristics, and percutaneous biopsy technique and complications were recorded. Analyses of per-tumor growth rate, per-procedure diagnostic rates, and per-patient disease-specific and metastasis-free survival were evaluated.

Results: The database search yielded 8 tumors (mean diameter 2.0 cm, range 1.0 to 3.1 cm) in 8 patients (median age 60.5 y, range 40 to 66 y; 6 women) who underwent percutaneous biopsies and had imaging available for review. All tumors (8/8) were solitary, well-defined, and hypoenhancing on post-contrast images. For those with available MR, 100% (5/5) demonstrated restricted diffusion. On unenhanced CT, 62.5% (5/8) were hyperdense. The mean tumor growth rate was 0.7 mm/y (range: -0.1 to 3 mm/y) with a median imaging follow-up of 83.4 months (range: 1.6 to 198.0 mo). Specific diagnosis of metanephric adenoma on the first percutaneous biopsy was found in 75% (6/8) of patients; with repeat biopsy in 2 patients confirming metanephric adenoma. Per-patient survival outcome after a median clinical follow-up of 151.8 months (range: 1.6 to 250.6 mo) showed 100% disease-specific and metastasis-free survival.

Conclusions: Metanephric adenomas are usually solitary, well-defined, and hypoenhancing masses on imaging, hyperattenuating compared with the renal parenchyma on noncontrast CT, and with restricted diffusion on MR. Image-guided percutaneous biopsy results of this tumor are reliable and safe.