Anatomical Pathology Interesting Case Conference

Discussion:

Renal Medullary Interstitial Cell Tumor is a type of Non-Renal cell tumor arising from medullary interstitial cells. It is typically benign, and mostly asymptomatic and is most often found incidentally on imaging, resection, or autopsy.

Grossly, it is seen as well-circumscribed, solitary or multiple homogenous gray-white nodules in the renal medulla. Most lesions are small, measuring <1 cm in diameter, although larger tumors up to 5 cm have been reported rarely.

Increasing tumor size is associated with reduced cellularity, disappearance of tubular architecture, and deposition of abundant eosinophilic stromal material., composed predominantly of type III collagen rather than amyloid.

Microscopically, it is seen as spindle or stellate cells in basophilic or fibrotic stroma peripherally surrounded by tubules. Entrapped renal tubules are commonly present at the periphery and may show cystic dilatation. It may contain amyloid deposits.

The tumor typically exhibits weak-to-moderate calponin expression and positivity for Oil Red O and Sudan Black B stains.

On CT, it is seen as non-enhancing noncalcified hypoattenuating solid mass within the renal medulla.

When identified as a discrete renal mass, surgical excision is generally curative, and recurrence or metastatic potential has not been reported.