Clear cell renal cell carcinoma with heterotopic bone formation: A case report and literature review

The phenomenon of heterotopic bone formation (osseous metaplasia) is defined as an abnormal ossification of non-skeletal tissues and does represent a rare occurrence in the renal cell carcinoma setting. We describe a case of a 40-year old man with bilateral renal cell carcinomas of the histological clear cell subtype, with the right-sided renal cell carcinoma demonstrating heterotopic bone formation, as well as the presence of intratumoral adipose tissue. The etiology of bone formation in a renal cell carcinoma is unclear, but possible explanations include a response to tissue ischemia and the expression of Bone Morphogenetic Protein 2. The detection of these rare morphologic variations is of paramount importance, not to be mistaken as sarcomatoid transformation and renal sinus fat invasion, which would advance the pathologic tumor stage and aggressiveness of the disease.