TFE3-rearranged renal cell carcinoma with tumour thrombus in the ureter mimicking invasive transitional cell carcinoma.

Renal cell carcinoma (RCC) occasionally breaches the urothelial tissue barrier and extends into the collecting system mimicking a transitional cell carcinoma. We present a case of a TFE3-rearranged renal cell carcinoma in a young patient that extended to fill the entire upper urinary tract leading to a pre-operative diagnosis of upper tract urothelial carcinoma (UTUC) based on CT imaging. A sequential algorithm of IHC/FISH assays led to the correct diagnosis of TFE3-rearranged RCC. Awareness of this potential misdiagnosis, particularly in young patients, can mitigate significant morbidity.