Laparoscopic partial nephrectomy was performed. Postoperative histology did not confirm malignancy or cellular atypia. The internal content of the cyst was confirmed as fibrinous debris likely due to prior internal hemorrhage. Granulation tissue, metaplastic bone formation and red bone marrow were also found focally. 

This cystic mass demonstrates that CEUS, multiphase CE-MRI, and CECT all have their limits in the characterization of complex renal lesions and it is always histopathology which has the final word. The avid enhancement caused by the internal granulation tissue and red bone marrow can hardly be expected, and from a radiological perspective this lesion was characterized properly. It has to be also noted that although exceedingly rare, extramedullary hemopoesis has been reported in cystic renal cell cancer too ¹.

I would like to emphasize that biopsy of this lesion preoperatively would have been a mistake as due to internal tissue heterogeneity sampling does not provide results to be relied upon. 

Subsequent follow-up so far showed no progression in the other complex cysts and the small left renal solid lesion, as of 2022 these have remained stable. 

Altogether we have to once again remind ourselves that some rare tumor mimickers can evade preoperative diagnosis even using the most thorough imaging workup.