Metastatic Clear Cell Renal Cell Carcinoma Identified in a Gastric Biopsy: Diagnostic Recognition in an Unusual Site.

Metastatic clear cell renal cell carcinoma (ccRCC) to the stomach is rare and may mimic other clear cell neoplasms when encountered in limited biopsy samples. We report a case in which a gastric biopsy provided the first histopathologic confirmation of metastatic ccRCC in a 65-year-old man who had not yet undergone tissue sampling of a suspected renal lesion. The biopsy showed nests of epithelioid cells with clear cytoplasm and delicate vasculature, and immunohistochemical (IHC) staining demonstrated strong positivity for paired box gene 8 (PAX8), renal cell carcinoma (RCC) marker, and CD10, supporting renal origin. Negative staining for cytokeratin 7 (CK7), cytokeratin 20 (CK20), caudal-type homeobox transcription factor 2 (CDX2), and thyroid transcription factor 1 (TTF-1) helped exclude primary gastric, gastrointestinal, and pulmonary neoplasms. Subsequent imaging confirmed a renal mass concordant with the IHC findings. This case underscores the importance of recognizing morphologic clues and using a targeted IHC panel when evaluating clear cell neoplasms in the gastrointestinal tract.