A Frozen Tale of Two Tumors in the Pancreas: An Intraoperative Diagnostic Challenge of Clear Cell Pancreatic Neuroendocrine Tumor Mimicking Metastatic Clear Cell Renal Cell Carcinoma

Abstract Introduction/Objective Clear cell pancreatic neuroendocrine tumor (PanNET) is rare, with very few cases reported in the literature, and is often found in patients with von Hippel-Lindau (VHL) disease. The differential for this entity includes ectopic adrenal tissue and metastatic clear cell neoplasms. These challenges during intraoperative consultation add to the complexity of diagnosing this condition. Distinguishing it from other pancreatic lesions requires careful examination and further ancillary testing. Methods/Case Report We report a case of clear cell PanNET mimicking metastatic clear cell renal cell carcinoma (RCC) in a 56-year-old woman with recently diagnosed VHL. She presented with renal masses and a pancreatic lesion with previous biopsy showing conventional NET. Intraoperative consultation during her pancreaticoduodenectomy identified a 1.3 cm well-circumscribed, golden-yellow solid mass. Further characterization was deferred to permanent sections given the diagnostic challenge of distinguishing between a PanNET and metastatic clear cell RCC. Permanent sections showed nests of clear cells with prominent nucleoli. The neoplastic cells were immunohistochemically positive for neuroendocrine markers synaptophysin, chromogranin and insulinoma- associated protein 1(INSM1), as well as pankeratin and PAX8, with a Ki-67% proliferation index <1%, consistent with low-grade clear cell PanNET. Clear cell RCC marker, CA-IX, as well as adrenocortical marker SF1, were negative. The combination of immunohistochemical stains is key for making the diagnosis. The previously biopsied conventional PanNET was also identified, which was positive for neuroendocrine markers and had a Ki-67 proliferation index <1%. Results (if a Case Study enter NA) NA Conclusion Given the rarity of this entity and its strong association with VHL, clear cell PanNET should be considered in the intraoperative consultation to inform clinical management. Furthermore, literature review indicates that metastatic RCC in the pancreas is exceptionally rare in VHL patients and typically of low nuclear grade, suggesting low metastatic potential. Thus, this case emphasizes the diagnostic challenges and the necessity of deferring to permanent sections with immunohistochemical work-up during intraoperative consultation or when assessing small biopsies.