- It is often difficult to distinguish Wilms tumors from nephrogenic rests based on imaging studies and percutaneous biopsies. The AREN0534 study uses the guideline that Wilms tumor with a single lesion 1 cm or greater in the contralateral kidney or multiple lesions (of any size) in the contralateral kidney should be treated on the synchronous bilateral Wilms tumor stratum; patients with an isolated lesion less than 1 cm in the contralateral kidney should be treated on the appropriate study for unilateral Wilms tumor OR on the unilateral Wilms tumor/contralateral nephrogenic rest stratum of this study if they have not undergone nephrectomy and are under one year of age.