Answer: Low grade myxoid distal nephron neoplasm (Henle loop tumor)

Histology: Histologic description: The tumor has a pushing border with the native kidney. It is characterized by an interconnecting tubular and spindled architecture, and a focally myxoid stroma. Tumor cells have bland, round nuclei and eosinophilic to vacuolated cytoplasm. Mitoses and necrosis are not identified.

Discussion: Differential diagnosis: Angiomyolipoma is suggested by the presence of spindled and epithelioid cells within a cytologically bland renal tumor. However, the current lesion lacks the fat and dysplastic vessels typical of angiomyolipoma, and the presence of tubular differentiation virtually excludes a diagnosis of angiomyolipoma. Renal medullary carcinomas are highly aggressive, medullary-centered neoplasms that occur in patients with sickle cell trait. These tumors are highly invasive, have rhabdoid cytology, and are associated with an extremely bleak prognosis. Survival is typically measured in weeks. Sarcomatoid renal cell carcinoma is suggested by the presence of both tubular differentiation and a spindle cell component; however, the current lesion lacks the invasive qualities, atypia, mitoses, and necrosis that is typical of sarcomatoid renal cell carcinomas.

The current lesion was first described by Srigley et al in abstract form as “Unusual Renal Carcinoma with Prominent Spindle Cell Change Possibly Related to the loop of Henle.” The first published description was by Parwani et al as “Low Grade Myxoid Renal Epithelial Neoplasm with Distal Nephron Differentiation.” These novel, rare renal neoplasms are characterized by the following main features:
1- female predominance,
2- circumscription and frequent renal medullary epicenter,
3- interconnecting tubular and spindled architecture within myxoid stroma,
4- low grade nuclei and eosinophilic to vacuolated cytoplasm,
5- ultrastructural features consistent with distal nephron origin (collecting duct or loop of Henle),
6- benign or indolent clinical course. Published immunohistochemical profiles of this tumor have been somewhat variable with cytokeratin 7 and EMA labeling most consistently. High molecular weight cytokeratin and Ulex labeling have been reported in some but not all cases.