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

<bold>Introduction and importance: </bold> 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. <bold>Case presentation: </bold> 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 7x6.5x6.5 cm showing intraluminal densities and heterogeneous content, and a lobulated partially exophytic renal mass measuring 4.5x5.5x4.5 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. <bold>Clinical discussion: </bold> 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. <bold>Conclusion: </bold> 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.