4.10 Metanephric Adenoma

•Gross features:

–The tumor is believed to arise from urothelium and is attached to the renal medulla, specifically the papillary region of the renal pyramids.

–From this location, it extends in a polypoid fashion into the collecting system.

–It is seldom more than 2–3 cm in diameter.

•Microscopic features:

–At microscopic analysis, the lesion consists of three basic components:

•An osteoid core

•Osteoblasts

•Spindle cells

–Mature osteoid elements tend to be more prominent in older patients.

•Clinical features:

–Hematuria is the usual presenting symptom.

–This is intermittent painless gross hematuria.

–In infants with gross hematuria and a calcified noninvasive mass in the pelvi-cal- ceal system, renal ossifying tumor should be considered in the differential diagnosis.

•Investigations:

–MRI or CT scan offers a good diagnostic guide.

–Abdominal ultrasound, CT-scan or MRI offers a good diagnostic guide.

–On abdominal ultrasound, the mass is echogenic with shadowing, and hydronephrosis may be present.

–Abdominal CT-scan shows a well-defined mass, often calcified, with poor enhancement.

–At imaging, the renal outline is usually maintained; however, filling defects with partial obstruction of the collecting system are often seen.

–Because of its location within the collecting system and its characteristic ossification, ossifying renal tumor of infancy may mimic a staghorn calculus, which would be exceedingly rare in the age group in which this lesion occurs.

•Treatment:

–The treatment is surgical excision.

–Open pyelostomy and tumor enucleation

–Partial nephrectomy

•The biologic behavior of ossifying renal tumor of infancy appears to be benign, with no reported cases of malignant change or spread.

•Surgical excision is curative.

4.10Metanephric Adenoma

4.10.1 Introduction

•Metanephric adenoma is also known as nephrogenic adenofibroma or embryonal adenoma.

•Metanephric adenoma has been described in the past under other names such as néphrome néphronogène, metanephroider Nierentumor and nephroblastomartiges Nierenadenom.

•The term metanephric adenoma was suggested by Brisigotti, Cozzutto et al. in 1992.

•It is a benign renal tumor that can occur at any age.

•It has been reported in patients as young as 15 months and as old as 83 years (Mean age at onset is around 40 years).

•It is more common in females.

•It is usually unilateral, and no bilateral cases have been reported.

•The presenting features include abdominal pain, hypertension, hematuria, a flank mass, hypercalcemia, and polycythemia.

•Polycythemia is more frequent in MA than in any other type of renal tumor.

•The tumor is more commonly calcified than any other renal neoplasm

•Grossly, MA tumors appear tan in color with multiple foci of hemorrhage.

•MA is composed of tightly packed uniform small epithelial cells with small regular nuclei, a high nuclei-to-cytoplasm ratio, and no mitotic figures.

•At histologic analysis, there is proliferation of spindle-shaped mesenchymal cells encasing nodules of embryonal epithelium. Numerous psammoma bodies may be seen.

•It is important to consider metanephric adenoma in the differential diagnosis of pediatric renal masses, since its benign nature allows local surgical resection with sparing of normal ipsilateral renal tissue.

•It is different from mesonephric adenoma.

•Surgery is curative.

4.10.2 Histopathology

•Gross features:

–The mean size of the tumor is 5.5 cm with a range 0.3–15 cm.

–The tumors can be located at upper pole, lower pole or mid-hilar region of the kidney.

–These tumors are well circumscribed but unencapsulated.

–On cut section, the tumor is tan pink, with possible cystic and hemorrhagic foci.

–Calcifications are uncommon, and only occur in approximately 20 % of cases.

•Microscopic features:

–Microscopically, metanephric adenoma can have an appearance similar to a nephroblastoma (Wilms tumours), or a papillary renal cell carcinoma.

–They show a uniform architecture of closely packed acinar or tubular structures of mature and bland appearance with scanty interposed stroma.

–The cells are small with dark staining nuclei and no nucleoli.

–Blastema is absent and glomeruloid figures are a striking finding.

–The lumen of the acini may contain otherwise epithelial infoldings or fibrillary material but it is quite often empty.

–Mitoses are absent.

–The tumor cells characteristically have round nuclear membrane, no nucleoli, and fine chromatin.

•Metanephric adenoma lacks the frequent gain of chromosomes 7 and 17 and losses of the Y chromosome that are seen in patients with papillary renal cell carcinomas.

4.10.3 Diagnosis

•Metanephric adenoma (MA) is a rare, benign tumor of the kidney.

•Metanephric neoplasms comprise a spectrum of kidney tumors containing renal epithelial or stromal cells or both.

•Although metanephric adenoma (MA) is a rare, benign neoplasm of epithelial cells, it is often difficult to distinguish this entity from other malignant neoplasms preoperatively.

•The differential diagnosis of renal MA includes papillary renal cell carcinoma and epithelial Wilms tumor.

•It is most important to distinguish MA from papillary renal cell carcinoma.

•Currently, neither ultrasound nor CT scans can reveal distinct features of MA.

•Ultrasound scans show a mass that is well defined and solid with both hyperechoic and hypoechoic regions.

•Doppler evaluation shows the lesion to be hypovascular.

•CT scans show a non-distinct mass with low attenuation on contrast studies, and small calcifications may be present. The lesion enhances less than normal renal parenchyma.

•Fine needle aspiration can be used as another less invasive method to diagnose MA, but it is not as accurate as nephrectomy.

•Add to this the fact that cytological diagnosis using fine needle aspiration can be difficult.

•Despite being a benign lesion, MA should be routinely resected in order to confirm the diagnosis and rule out papillary renal cell carcinoma.

4.10.4 Clinical Features

•Metanephric adenoma (MA) is a rare neoplasm, accounting for 0.2 % of adult renal epithelial neoplasms.

•Metanephric adenoma can occur at any age.

•It has been reported in patients as young as 15 months and as old as 83 years.