Undiagnosed coexisting conditions, such as sarcoidosis, can result in false positive findings on CT and PET/CT scans when evaluating patients with renal masses for metastatic disease. The levels of radiotracer activity at each site should be evaluated because significant differences in activity may suggest separate disease processes.

From the manuscript

Co-existing Sarcoidosis Confounds the Staging of Bilateral Renal Cell Carcinoma

Abstract
We present a case in which the undiagnosed condition of sarcoidosis complicated the staging of bilateral, subtype-discordant renal cell carcinoma. Initially thought to have metastatic renal cell carcinoma based on computed tomography imaging and referred for immunotherapy, a positron emission tomography/computed tomography scan demonstrated different levels of radiotracer activity in the primary site and the presumed pulmonary metastatic sites. The patient underwent bilateral partial nephrectomies and was ultimately diagnosed with stage T1 bilateral renal cell carcinoma and sarcoidosis. This case highlights the need to consider concurrent medical conditions that can lead to false positive results when evaluating for metastatic disease with imaging studies as well as the importance of evaluating the levels of radiotracer activity between different sites.