These tumors were first described in 1980 by Pag`es Granier and French literature. Several reports of similar lesions described in a variety of names then arose and was confirmed as a distinct entity. Include metanephric adenoma (predominantly epithelial), metanephric adenofibroma and metanephric stromal tumor (gastrointestinal stromal tumor) which is a pediatric tumor identical to the stromal component of metanephric adenofibroma.
Most clinical and pathological features of metanephric adenoma outlined around mid-1990 in two large series. metanephric adenofibroma was identified then as a biphasic tumor with epithelial and stromal elements and occurs mainly in children and young adults. metanephric tumors as a group are highly cellular benign epithelial tumors, with a close relationship with the PT and conceptualized by some as the benign end, as opposed to a spectrum of tumors that also includes WT and its malignant counterpart.
Pathology
These tumors vary in diameter from 3-15 cm. They are usually single-center, clearly circumscribed without a capsule. The cut surface is gray to yellow with focal hemorrhage, cystic changes, and necroses are uncommon. Histologically composed of well-packed small, round acini. Half of the tumors containing papillary structures that resemble primitive glomeruli. Psammoma bodies are frequently present. No blast elements are present. The stroma may be discrete or edematous. The cells are generally cubic-like monotone, scant cytoplasm and small nuclei, uniform with inconspicuous nucleoli. metanephric adenofibroma is composed of nests of epithelial elements similar to metanephric adenoma included in the bands and sheets of fibroblast-like spindle cells.
The proportion of spindle cells and epithelial components in these tumors varies. metanephric stromal tumors, as its name implies, is very similar to the stromal component of metanephric adenofibroma. The immunohistochemical profile includes positive WT1 and CD56 and epithelial membrane antigen and CK7 negative. The differential diagnosis usually includes papillary renal cell carcinoma (PRCC) and epithelial predominant WT. PRCC is more common in men, tends to be multifocal, with a pseudocapsule, and a different immunohistochemical profile (WT-1 negative, EMA, and CK7 positive.)Epithelial predominant WT is usually seen in younger patients, with a pseudocapsule, and more atypical cells with abundant mitotic activity. Immunohistochemistry, metanephric tumors and WT share resemblance - both are positive for WT-1 antigen and negative for EMA and CK7. However, CD56 is positive in metanephric tumors and negative in WT.
Clinical Presentation
Metanephric adenoma can occur in children and adults, however, is predominant in the fifth and sixth decades of life, with a distinct female preponderance (ratio between women and men 2: 1). metanephric adenofibroma is usually seen in children and young adults aged 5 months to 36 years with a male preponderance. Metanephric stromal tumors are seen mainly in children, if adults rarely reported. These tumors as a group comprise less than 1% of renal cell neoplasms. Most of these cases are discovered incidentally during imaging studies and differential diagnosis of incidental hematuria.
Radiographically, the metanephric adenoma appears as a hypovascular tumor protruding extrarenally.When symptomatic, can cause abdominal pain and hematuria. Erythrocytosis has been reported in patients at presentation.
Treatment and Prognosis
These tumors are benign, with the exception of a few case reports in question. If the metanephric adenoma is suspected on clinical findings, it is important to obtain an intraoperative diagnosis in order to avoid excessive resection. Erythrocytosis associated with these tumors resolved after complete resection. No local recurrence or distant metastasis has been reported metanephric stromal tumors. WT has been reported to have emerged in metanephric adenofibroma and metanephric stromal tumor, indicating a possible common origin of these entities. Renal angiodysplasia associated with these injuries may cause morbidity secondary to vascular complications. Resection without adjuvant chemotherapy is the preferred modality of treatment.