Supplementary Material for: Cytological features of the emerging entity Papillary Renal Neoplasm with Reverse Polarity: a case report highlighting the relevance of fine-needle aspiration in renal masses

Introduction Papillary Renal Neoplasm with Reverse Polarity (PRNRP) is an emerging entity characterized by distinctive histomorphological and molecular features. Often identified incidentally in imaging studies, its accurate diagnosis is essential considering its favorable prognosis. However, its cytological features need further characterization, since only one single publication with two cases diagnosed in fine-needle aspiration biopsies have been reported to date. Case Presentation We report the case of a 70-year-old man recently diagnosed with primary lung adenocarcinoma. Fine-needle aspiration biopsy of a 15 mm nodule in the left kidney revealed cytological features consistent with a papillary neoplasm with eosinophilic features. Immunocytochemistry showed positivity for PAX-8, CK7, and GATA3, and negativity for TTF-1. Next-generation sequencing (NGS) identified a KRAS G12V mutation. Altogether, these findings enabled the diagnosis of PRNRP. The patient underwent conservative management of the renal tumor, which did not recur at eight months since diagnosis. Conclusion PRNRP is a recently described tumor which can be managed conservatively. We describe the third case of PRNRP diagnosed on fine-needle aspiration biopsy, which enabled several diagnostic studies, including immunocytochemistry and NGS confirmation. With this case we highlight the increasing role of fine-needle aspiration biopsy in the clinical management of a subset of patients with small renal masses.