Outcome analysis of stage I epithelial-predominant favorable-histology Wilms tumors: A report from Children's Oncology Group study AREN03B2

Background: Stage I epithelial-predominant favorable-histology Wilms tumors (EFHWTs) have long been suspected to have an excellent outcome. This study investigates the clinical and pathologic features of patients with stage I EFHWTs to better evaluate the potential for a reduction of chemotherapy and its associated toxicity. Methods: All patients registered in the Children's Oncology Group (COG) AREN03B2 study between 2006 and 2017 with stage I EFHWTs were identified. EFHWTs were defined as tumors with at least 66% epithelial differentiation, regardless of the degree of differentiation. Clinical information was abstracted from COG records. Event-free survival (EFS) and overall survival (OS) were calculated and compared between groups based on age and therapy. Results: The 4-year EFS rate was 96.2% (95% confidence interval, 92%-100%), and the OS rate was 100%; EFS and OS did not statistically significantly differ with the age at diagnosis (<48 vs ≥48 months; P =.37) or treatment (EE4A vs observation only; P =.55). Six events were reported. Three patients developed contralateral tumors and did not otherwise relapse; none of these had nephrogenic rests or a recognized predisposition syndrome. Three patients developed metastatic recurrence; all 3 had received EE4A as their primary therapy after nephrectomy. Conclusions: These findings demonstrate an excellent outcome for stage I EFHWTs with >95% EFS and OS. These data support the utility of investigating the treatment of stage I EFHWTs with observation alone after nephrectomy.