TSC/MTOR mutated renal cell carcinoma with leiomyomatous stroma is a distinct entity: a comprehensive study of 12 cases

The 2016 World Health Organization (WHO) Classification of Tumors of the Urinary System includes renal cell carcinoma (RCC) with leiomyomatous stroma (RCC-LS) as a provisional category. Recent studies have shown that this category includes at least 4 subtypes: clear cell (CCRCC), clear cell papillary renal cell tumor (CCPRCT), ELOC (TCEB1) mutated, and a subtype of RCC with TSC/MTOR mutations. The most recent 2022 World Health Organization (WHO) Classification of Tumors of the Urinary System includes ELOC mutated RCC-LS as a distinct entity but does not address any other renal tumors with smooth muscle stroma. We reviewed >500 cases of RCC with clear cell phenotype and identified 12 cases that exhibited prominent smooth muscle stroma, of which 4 of the cases had been previously reported. Review of the H&E revealed that all of the tumors were circumscribed with nested, solid, tubular, and tubulopapillary architecture. The epithelium was intimately embedded in the rich smooth muscle stroma. WHO/ISUP grade corresponded to grade 3 and 4. Nuclei were randomly distributed and the cytoplasm had predominantly clear and occasionally flocculent appearance. Immunohistochemically, all the cases showed membranous CAIX staining, although the pattern was combined cup and box-shaped. CK7 was positive in all cases ranging from 25% to 100% of cells. Membranous and apical staining of CD10 was present in all cases. Next generation sequencing (NGS) of these cases identified mutations in TSC1 (n = 4), TSC2 (n = 3), and MTOR (n = 4) with one case exhibiting loss of TSC1. This descriptive study, although small, demonstrates the difficulty in applying the current WHO provisional criteria at a single institution. Given the heterogeneity seen with these cases, we suggest following up an immunohistochemical panel of CAIX, CK7, and CD10 with molecular diagnostic studies to assist in the diagnosis of TSC/MTOR associated RCC-LS, which we believe is a distinct entity.