Renal Cell Carcinoma with Fibromyomatous Stroma: A Case Report.

BACKGROUND Renal cell carcinoma with fibromyomatous stroma (RCC FMS), which includes positivity for elongin C/transcription elongation factor B mutation, is a relatively new World Health Organization (WHO) entity in the classification of renal tumors, with less than 20 reported cases in total. Clinically, the tumors present as solitary renal cortical masses, pelvic pain, or hematuria. These tumors lack VHL mutations, hypermethylation, or loss of heterozygosity at 3p, which are characteristics of clear cell renal cell carcinoma. CASE REPORT We report a case of renal neoplasm in a 59-year-old male patient who initially presented with hematuria. Radiological examination showed a 2-cm rounded mass located at the upper pole of the right kidney. Histological examination revealed a renal neoplasm formed of nests, solid areas, and alveolar arrangement of WHO/International Society of Urological Pathology grade 1 epithelioid cells, transected by thick fibromuscular bands. The tumor cells had voluminous clear cytoplasm, well-defined cell borders, and central, rounded, non-atypical nuclei. Immunohistochemically, the tumor was positive for CK7, AMCR, PAX-8, and CD10, while the stromal muscle bundles were positive for smooth muscle actin and vimentin. All of the mentioned features are characteristic of RCC FMS. CONCLUSIONS This case report presents RCC FMS as a new subtype of RCC, characterized by unique morphologic, immunohistochemical, and molecular features that set it apart from more prevalent types of renal epithelial tumors, such as clear cell RCC, papillary RCC, and clear cell papillary RCC. Due to its rarity and good prognosis compared with other renal tumors, attention should be paid when dealing with renal epithelial tumors that exhibit the usual features of other epithelial tumors but with abundant fibromyomatous stroma.