Cystic nephroma with heterotopic bone formation: a pathological and radiological pitfall

This case illustrates heterotopic bone formation in a patient with cystic nephroma (CN) of the kidney. Renal CN may be difficult to discern clinically and radiologically from ‘Multilocular Cystic Renal Cell Neoplasm of Low Malignant Potential (MCRCNLMP)’ or a clear cell renal cell carcinoma with extensive cystic change. In both of the latter, dystrophic calcification or ossification is not uncommon. The formation of heterotopic bone in CN/MEST is rare and should not be confused with sarcomatous transformation on histology. Radiologically, the presence of central calcification increases the suspicion of a cystic renal cell carcinoma, a potential diagnostic pitfall.