Renal Adenosarcoma Mimicking a Malignant Pelvocalyceal Tumor: An Interesting Imaging Case

We report a rare case of primary renal adenosarcoma in a 26-year-old woman presenting with right flank pain. Contrast-enhanced computed tomography demonstrated a large, mixed solid and cystic mass confined to the renal pelvocalyceal system, closely mimicking a malignant renal tumor. Histopathologic examination revealed a biphasic tumor with phyllodiform architecture. Immunohistochemistry showed benign epithelial positivity for cytokeratin and focal malignant stromal positivity for smooth muscle actin and CD10; however, the tumor was negative for CD99, Wilm's tumor protein, SS18-SSX, BCOR, and estrogen/progesterone receptors. These findings led to the diagnosis of primary renal adenosarcoma. This case highlights the diagnostic challenge of distinguishing this rare tumor from more common renal malignancies and underscores the importance of imaging-pathologic correlation.