TFE3-rearranged RCC with osseous metaplasia found on bone mass densitometry

Renal cell carcinoma (RCC) is the most common primary renal malignancy in patients between the ages of 50 and 70. A rare described variant of RCC is transcription factor for immunoglobulin heavy-chain enhancer 3 (TFE3) rearranged RCC. Osseous metaplasia, which refers to the occurrence of normal bone tissue in soft tissue, has been observed in all subtypes of renal cell carcinoma (RCC); however, only three previous case reports have documented the occurrence of osseous metaplasia in TFE3-rearranged RCC.

We present a case of a 65-year-old woman presenting with an incidentally discovered calcified Bi-lobed renal mass detected on bone densitometry composed of a calcified thick-walled cyst measuring 7×6.5×6.5 cm showing intraluminal densities and heterogeneous content, and a lobulated partially exophytic renal mass measuring 4.5×5.5×4.5 cm. The patient underwent robotic-assisted radical nephrectomy confirming the diagnosis of RCC with osseous metaplasia extending into the pelvic calyces and renal sinus fat implicating a pathological stage of T3a.

TFE3-rearranged RCCs represent a rare sub-classification in adult RCCs. It may be associated with unfavorable prognosis and aggressive patterns of disease in the presence of osseous metaplasia.

This is the first case in the literature of TFE3-rearranged RCC with osseous metaplasia on bone mass densitometry scan and the fourth case of TFE3-rearranged RCC with osseous metaplasia. The patient is to be treated as a high-risk patient and to be monitored closely for recurrence of malignancy, as indicated in the EAU guidelines.