Mucinous Borderline Tumors of the Pelvicalyceal System: Report of Two Tumors With Molecular Findings and Review of Literature.

Mucinous borderline tumors of the pelvicalyceal system are exceedingly rare tumors with very little information regarding their clinicopathological characteristics and molecular underpinnings in the published literature. Herein, we report two mucinous borderline tumors and review the literature pertaining to these tumors. The patients were women who presented with flank pain without any history of nephrolithiasis. Abdominal computed tomography (CT) showed multiloculated cystic lesions without mural nodules involving the kidney (Bosniak category III). The nephrectomy specimens showed multiloculated cysts filled with mucoid material, without any solid areas. On microscopic examination, the cystic spaces were lined predominantly by a single layer of mucinous epithelium with variable foci of epithelial stratification. Extensive sampling of the tumors did not show any evidence of invasion. The tumor cells were positive for keratin 7, keratin 20, and CDX2 and were negative for urothelial markers. Molecular analysis performed on one of the tumors showed loss of function mutation of SMAD4 p.S32fs. The presence of SMAD4 alterations alludes to the potential role of the SMAD4-TGFß pathway in the pathogenesis of this tumor. On clinical follow-up, both patients are well with no evidence of disease. Our case report expands on the clinical characteristics of these rare tumors and provides insight into the previously unreported molecular landscape of this tumor.