Metanephric adenoma diagnosed on biopsy in an infant: a case report

Metanephric adenomas (MAs) are rare benign neoplasms of the kidney. They derive from the renal residual tissue during embryonic development. These tumors occur predominantly in middle-aged women with few cases reported in children [1, 2].

Because of the lack of specific clinical and radiographic characteristics, they are frequently misdiagnosed preoperatively as malignant tumors of the kidney. The diagnosis is mostly established based on histological examination after an unnecessary radical nephrectomy. However, accurate diagnosis made preoperatively is of great importance as MA should be treated conservatively with nephron-sparing surgery. Herein, we describe the case of an infant who underwent conservative treatment for a metanephric adenoma diagnosed by preoperative fine-needle biopsy.

Management of renal masses in children can be challenging, especially when lacking specific radiologic features. MA is a less common benign condition in children; its similarity to other malignant renal tumors makes establishing a preoperative diagnosis very important to guide the adequate surgery technique. To our knowledge, our patient is the youngest case of a child with MA who underwent nephron-sparing surgery [3].

Clinically, MA may present with hematuria, flank pain, hypertension, or abdominal mass [3‐5]. However, as in this case, metanephric adenomas are usually asymptomatic lesions, detected incidentally on imaging studies performed for other indications. However, imaging can only offer some general clues that can be used only to suspect the diagnosis of MA. CT is the main imaging method used for diagnosis, but there is no specific radiological feature of MA. Generally, MA is consistently well defined and mostly has an intact capsule with no distinct attenuation patterns. It is mostly spontaneous and slightly hyperdense in comparison with normal renal parenchyma [6]. Calcifications can be observed in 20% of cases [7]. Delzongle et al. [5] concluded that there was no correlation between CT or magnetic resonance imaging (MRI) findings and histological characteristics of MA and that imaging cannot be specific at all. Those findings led to the approach of preoperative biopsy to improve diagnostic accuracy and spare the children an unnecessary nephrectomy. Amodio et al. [3] reported that MA can be confused with other neoplasms such as sarcoma or carcinoma and is hardly detectable without preoperative biopsy. Fine-needle biopsy in the management of small renal masses is still rather controversial. In fact, surgical management based on CT imaging without pretreatment biopsy is considered appropriate in most centers. However, 20% of surgically removed small renal tumors have proven to be benign [8]. Thus, it is necessary for current practices to be reevaluated to avoid overtreatment. Actually, fine-needle percutaneous biopsy is considered a safe and effective procedure in the management of small renal masses [9].

The final diagnosis is confirmed by postoperative histopathological examination, which accurately describes all the characteristics of the tumor [10]. Microscopically, the classic appearance of MA is a cellular blue tumor composed of tightly packed tubules, long branching and angulated ducts, and abortive glomeruli. Tumor cells have a scant cytoplasm and small nuclei with no nucleoli. Mitotic figures are very rare or absent. Stroma is scant and can be edematous and occasionally look scar-like. Psammomatous calcifications can be abundant [11]. Differential diagnoses include papillary renal cell carcinoma type 1, solid variant and adult epithelial predominant nephroblastoma. Papillary renal cell carcinoma, type 1 is a PAX8+ , vimentin+ , CK7+ , AMACR+ , WT1−, CD57−, BRAF− tumor as opposed to metanephric adenoma that is positive for WT1, CD57, and BRAF but negative for CyK7. Adult nephroblastoma is positive for WT1 but negative for CD57 and BRAF [11]. In our case we did not test BRAF due to a lack of this antibody in our Anatomopathology department.

Partial or radical nephrectomy is the mainstay of treatment for MA. Radical nephrectomy still remains the treatment of choice when the preoperative biopsy is not conclusive.

The possibility of nephron-sparing surgery in MA in the pediatric population has previously been addressed in literature [12]. Partial nephrectomy or thermoablative procedures are also recommended, according to the American Urological Association Guidelines [13].

The follow-up is not well defined, but should be done with clinical and radiological examination according to Liniger et al. [10].

MA is rare in children, and even rarer in infants. Its preoperative diagnosis is challenging as it lacks specific imaging features. Giving the high probability of subjecting infants to an unnecessary radical nephrectomy, preoperative biopsy can be a safe procedure that guides the management plan toward the more conservative nephron-sparing surgery.