Discussion:

Mixed epithelial and stromal tumor (MEST) of kidney was first described by Michal and Syrucek in 1998.

These lesions are a part of MEST family tumors, with adult cystic nephroma at the opposite end of the spectrum.

MEST are rare and constitute 0.20-0.28% of renal neoplasms.

Usually affects middle-aged/peri-menopausal women between 36-80 years of age.(Mean age-46 years, F:M =6:1)

Unknown etiology, possible role of hormones in pathogenesis.

Usually benign without recurrence or metastasis, rare malignant transformation has been reported in few cases.

Clinical features include palpable abdominal mass, hematuria, flank pain and urinary tract infection.

~25% - asymptomatic and incidental finding.

On computed tomography, they mimic cystic nephroma and multilocular cystic renal cell carcinoma.

Grossly, tumor can show a wide range of size, are well-circumscribed with solid and cystic components.

Microscopically, the tumor shows biphasic growth pattern with epithelial and stromal (mesenchymal) components.

Epithelial component consists of cysts lined by flat, cuboidal, columnar, hob-nail, urothelial-like, clear cells or ciliated cells. The cells show minimal cytologic atypia.

Stroma can be pauci-cellular to hyper-cellular, shows spindle cells. Areas may show smooth muscle differentiation, fibroblastic or myofibroblastic differentiation, or ovarian type stroma with luteinization.

On immunohistochemistry, the epithelial cells are positive for cytokeratins, PAX8, PAX2 and GATA3 and the mesenchymal component is positive for ER, PR, CD10, inhibin, FOXL2, SMA, Desmin, Caldesmon and negative for HMB45 and Cd34.

The differential diagnosis includes adult cystic nephroma, multi-locular cystic renal cell neoplasm of low malignant potential, angiomyolipoma with epithelial cysts, renomedullary interstitial cell tumor and mesoblastic nephroma.

Adult cystic nephroma is a part of MEST family and shows overlapping features. It can be differentiated from MEST by its lack of solid component. Multi-locular cystic renal neoplasm of low malignant potential does not have cellular stroma. The cysts are lined by large clear cells with abundant cytoplasm. Angiomyolipoma is a PEComa (perivascular epithelioid cell tumors) and is positive for melanocytic markers like HMB45 and Melan A. Renomedullary interstitial tumors are rare incidental tumors, usually of small size and show entrapped renal tubules and weak positivity for ER and PR. Mesoblastic nephroma is seen in younger age group and is negative for ER and PR.

In malignant transformation, there is increased mitoses, necrosis and cytologic atypia and rhabdoid, rhabdomyosarcomatous, and chondrosarcomatous differentiation may be seen.

Nephron-sparing surgery is the standard treatment and shows favorable prognosis.

Subset of these patients may require continuous post-operative follow up to rule out possible malignant transformation.