Non-Wilms Renal Tumours in Children: The Republic of Ireland Experience.

Background: Non-Wilms renal tumours (NWRTs) are rare paediatric malignancies and account for a small but clinically significant proportion of childhood renal cancers. Due to their low incidence and biological heterogeneity, outcome data are limited, and management is largely extrapolated from international collaborative protocols. No national data describing the incidence and outcomes of NWRTs in children in the Republic of Ireland (ROI) have previously been published. Objective: To determine the incidence, treatment strategies, and survival outcomes of NWRTs in children in the ROI. Methods: A retrospective cohort study was conducted of all children under 16 years of age with histologically confirmed renal tumours diagnosed and treated at Children's Health Ireland (CHI) at Crumlin between January 2005 and December 2025. As CHI Crumlin is the single national paediatric oncology centre in the ROI, this cohort represents national case ascertainment for the study period. A total of 143 paediatric renal tumours were identified; Wilms tumours (n = 118) were excluded, leaving 25 children (17.48%) with NWRTs for analysis. No cases of bilateral renal tumours were identified. Histological subtypes included renal cell carcinoma (RCC), clear cell sarcoma of the kidney (CCSK), congenital mesoblastic nephroma (CMN), malignant rhabdoid tumour of the kidney (MRTK), and anaplastic sarcoma of the kidney. Demographic characteristics, treatment strategies, and survival outcomes were analysed. Results: Twenty-five children with NWRTs were identified: CCSK (n = 9), RCC (n = 7), CMN (n = 6), MRTK (n = 2), and anaplastic sarcoma of the kidney (n = 1). At a median follow-up of 107.9 months (range 4.5-181.3 months), overall survival for the cohort was 76%. Overall survival by histology was 100% for CMN, CCSK and anaplastic sarcoma, 43% for RCC, and 0% for MRTK. Treatment strategies varied by histology, with 68% undergoing upfront surgery, 32% receiving neoadjuvant chemotherapy, 60% receiving adjuvant systemic therapy, and 44% receiving radiotherapy. Tumour recurrence occurred in 4/25 patients (16%), confined to the RCC (3) and CMN (1) subgroups. Seven Event-Free Survival events were observed, comprising three RCC relapses and one RCC progression, one CMN relapse, and two MRTK progression-related deaths. No recurrences occurred in CCSK. Conclusions: NWRTs comprised 17.5% of all paediatric renal tumours diagnosed nationally during the study period and demonstrated marked heterogeneity in outcomes according to histological subtype. CMN showed excellent survival with six out of seven requiring surgery alone, whereas MRTK remained associated with dismal outcomes despite multimodal therapy. These national data support histology-driven, risk-adapted management and highlight the importance of continued international collaboration to improve outcomes in NWRTs.