Papillary renal neoplasm with reverse polarity (PRNRP) is a recently defined entity previously classified as papillary renal cell carcinoma (PRCC). Unlike PRCC, PRNRP follows an indolent course with a favorable prognosis and is often detected incidentally. Here, we present a case of an incidental renal tumor diagnosed as PRNRP following partial nephrectomy.

A 65-year-old lady was found to have an incidental left renal nodule during a health checkup in July 2024. She was asymptomatic and visited our Urology clinic for further evaluation. Pelvic CT on July 22, 2024 revealed a 1.8×1.7 cm hypo-enhancing nodule, suspicious for RCC. After discussion, she opted for surgery. Left robotic-assisted retroperitoneal partial nephrectomy was performed on 2024/09/04. The total ischemic time was 19 minutes. During the surgery, a 1.84 x 1.75 x 1.90 cm tumor, confined by capsule, was identified without invasion of the sinus or collecting system. The majority of the lesion was located at the lower pole of the left kidney. Post-operative course was uneventful and the patient was discharged on post-operative day 5. Pathological analysis reported a Papillary renal neoplasm with reverse polarity of the left kidney, with papillary proliferation lined by eosinophilic cells with nuclei aligned along the luminal border. immunohistochemical staining revealed that the tumor cells were positive for GATA3, CK7, PAX8, and GPNMB, focally and weakly positive for AMACR, while being negative for CAIX. A follow-up CT scan five months postoperatively showed no evidence of local recurrence or metastasis and the patient was kept under follow-up.

Papillary renal neoplasm with reverse polarity (PRNRP) is a recently identified subtype of papillary renal tumors with distinct histological and immunohistochemical characteristics. First described in 2019, PRNRP is characterized by papillary structures lined by a single layer of uniform tumor cells with apical nuclei and basophilic cytoplasm, distinguishing it from conventional papillary renal cell carcinoma (PRCC). [1] Unlike type 1 and type 2 PRCC, PRNRP lacks significant nuclear atypia and aggressive features, indicating a favorable prognosis. [2] Immunohistochemically, PRNRP exhibits strong GATA3, CK7, and AMACR expression, with weak or negative staining for CD10 and vimentin. Molecular studies have identified recurrent KRAS mutations, differentiating PRNRP from other papillary renal tumors, which often harbor MET or FH alterations. [1] This unique molecular profile supports its classification as a distinct renal neoplasm. Clinically, PRNRP follows an indolent course, with no reported cases of metastasis or recurrence. Most cases are detected incidentally during imaging for unrelated conditions, and surgical resection remains the standard treatment. The primary differential diagnoses include conventional PRCC, oncocytic PRCC, and metanephric adenoma. Accurate diagnosis is essential to avoid unnecessary aggressive treatment. [1]

This case report underscores the rarity and uniqueness of papillary renal neoplasm with reverse polarity. This patient, presented with an incidentally identified renal tumor, was ultimately diagnosed as papillary renal neoplasm with reverse polarity after surgical intervention. The successful resection with no recurrence at six months highlights the importance of the effectiveness of surgical management and its relatively indolent nature. Reference [1] Chang HY, Hang JF, Wu CY, Lin TP, Chung HJ, Chang YH, Pan CC. Clinicopathological and molecular characterisation of papillary renal neoplasm with reverse polarity and its renal papillary adenoma analogue. Histopathology. 2021 Jun;78(7):1019-1031. [2] Han H, Yin SY, Song RX, Zhao J, Yu YW, He MX, Wang H. Papillary renal neoplasm with reverse polarity: an observational study of histology, immunophenotypes, and molecular variation. Transl Androl Urol. 2024 Mar 31;13(3):383-396.