Pelvicalyceal system (PS) involvement by renal cell carcinoma (RCC) is staged as pT3a disease (American Joint Committee on Cancer, AJCC, 8^th edition). As papillary RCC (PRCC) has been infrequently represented in studies looking at the prognostic impact of PS involvement, we reviewed our institutional cohort of 8,225 cases for PS involvement by PRCC. Nine such cases were subjected to histopathologic review and immunohistochemistry. FISH for TFE3/TFEB alterations was performed if indicated. One case each (1 of 9, 11%) was classified as TFE3-rearranged and FH-deficient RCC. The majority were high grade (WHO/ISUP grade 3: 8 of 9, 89%) or had features of aggressive disease, including hilar fat (6 of 9, 67%) and regional lymph node involvement (5 of 7, 71%). One low grade 3.3 cm tumor with isolated PS involvement with a germline heterozygous FH p.Lys477dup alteration with retained FH, lack of increased 2SC, BRAFV600E IHC positivity and lack of trisomy 7/17 on chromosomal microarray was identified, arguing against an FH-deficient and conventional PRCC. Our study shows that PS involvement by renal neoplasia with papillary architecture is a rare event. Aside from PRCC, it is important to note that these may include other aggressive and non-aggressive subtypes of renal neoplasia with papillary architecture. One case of isolated PS involvement by a low grade, non-invasive tumor that we refer to as nephrogenic papillary neoplasm, was identified. At present, there is insufficient data to stage such tumors as pT3a (AJCC, 8^th edition) and additional studies are needed to address this question.

Keywords: BRAF p.V600E mutated RCC; FH p.Lys477dup; Nephrogenic papillary renal neoplasia; Papillary renal cell carcinoma; Pelvicalyceal System; Staging.