[CD117-positive eosinophilic renal cell tumors with uncertain classification: a clinicopathological and molecular...

Objective: To investigate clinicopathological and molecular genetic characteristics of CD117-positive eosinophilic renal cell tumors (ERCTs) with unusual morphological and immunophenotypic features. Methods: Formalin-fixed, paraffin-embedded tissues from 10 cases (9 cases from Xiangya Hospital, Central South University and 1 case from Bishan Hospital of Chongqing Medical University) of diagnostically challenging CD117-positive ERCTs between January 2017 and October 2024 were collected. Histological reviews were performed on HE-stained sections, followed by immunostaining and whole-exome sequencing (WES). Results: The 10 patients were composed of 4 males and 6 females, with ages ranging from 29 to 57 years, median 49.5 (36.8, 51.8) years. The sizes of tumors ranged from 2.5 to 6.0 cm, median 4.8(2.9,5.2) cm. All 10 ERCTs were composed of variably eosinophilic cells and characterized by prominent morphological features including exclusively eosinophilic (2 cases), focal chromophobe-like (3 cases), prominent nested (2 cases), prominent flocculent cytoplasm (1 case), a collision of renal oncocytoma (RO)/chromophobe renal cell carcinoma (ChRCC) (1 case), and diffusely degenerative atypia (1 case). Immunohistochemically, a subset of 10 tumors variably expressed CK7 (7/10) and vimentin (3/10), while they were all positive for CD117 (10/10), PAX8 (10/10), SDHB (10/10), and FH (10/10) and negative for CAⅨ (10/10) and 2SC (10/10). The Ki-67 proliferation index ranged from 1% to 5%. WES identified a GNAS mutation in one case of the RO/ChRCC collision tumor, while no characteristic mutations of other renal cell tumor types were detected in the remaining 9 cases. The analysis of copy number variations revealed complex karyotypic alterations in 4 tumors, harboring various gain of chromosomes 4, 5, 7, 12, 13, 15, 16, 18, and 22, with 3 cases showing variable loss of chromosomes 1, 2, 6, 10, 13, and 17. These 4 cases were molecularly classified as eosinophilic ChRCC. The remaining 6 cases, including 2 cases with a normal diploid karyotype and 4 cases with slight karyotypic alterations, were molecularly classified as 5 ROs and 1 RO-dominant RO/ChRCC collision tumor. Finally, the original diagnosis was retained in 4 cases and revised in 6 cases. Conclusions: CD117-positive ERCTs with uncertain classification may exhibit various morphological overlaps, non-classic histological features, and aberrant immunophenotypes. Combined immunostaining of CK7, CD117, vimentin, SDHB, FH, and 2SC can greatly help discriminate among these tumors and their mimics. When the diagnosis is challenging based only on morphology and immunohistochemistry, molecular genetic tests may be useful.