Nephrogenic Rests/Nephroblastomatosis in Patients With Unilateral Wilms Tumor Are Not Associated With an Increased Risk of Relapse: An Analysis of Patients Treated on the SIOP‐WT‐2001 Protocol in the SIOP‐UK‐CCLG and SIOP‐GPOH Studies (2001–2022)

INTRODUCTION: Nephrogenic rests (NRs) and nephroblastomatosis (NBM) are precursor lesions for development of Wilms tumor (WT). Their association with the risk of relapse has not been properly assessed, partly due to misunderstanding of their diagnostic criteria and terminology. PATIENTS AND METHODS: Retrospective data analysis of patients with WT with and without associated NRs/NBM diagnosed and treated according to the International Society of Paediatric Oncology-WT-2001 protocol in the United Kingdom (UK) Children's Cancer and Leukaemia Group (UK-CCLG) and Gesellschaft für Pädiatrische Onkologie und Hämatologie (GPOH) groups' participation in the WT trials and studies (2001-2022). RESULTS: There were 1465 (59.5%) patients with no NRs (Group A), and 996 (40.7%) with NRs (Group B); the latter included 678 (68.1%) patients with perilobar NRs (PLNRs) (Group C), 250 (25.1%) with intralobar NRs (ILNRs) (Group D), and 68 (6.8%) with combined PL/ILNRs (Group E). There was no significant difference in relapse-free survival (RFS) and overall survival (OS) between Groups A-D. Univariate Cox regression analyses showed no significant prognostic impact on RFS or OS of age at diagnosis (only RFS), local stage, metastatic disease, and histology and no impact on NRs. Multivariate Cox regression analysis, including age, sex, histology, local stage, metastatic disease, and NRs, showed no significant effect on either RFS or OS for Group A versus B, Group A versus C, or Group A versus D. CONCLUSIONS: The study showed that the presence of NRs/NBM in unilateral WT is of no significance for RFS and OS, and therefore, these patients should not be treated or followed up differently from patients with WT without NRs/NBM.