Imaging Characteristics of Nephrogenic Rests Versus Small Wilms Tumors: A Report From the Children's Oncology Group Study AREN03B2.

Distinguishing nephrogenic rests from small Wilms tumors can be challenging. This retrospective study was performed to determine if imaging characteristics can be used to distinguish nephrogenic rests from Wilms tumors. All cases of pathologically confirmed nephrogenic rests and Wilms tumors smaller than 5 cm in maximum dimension on imaging in patients younger than 5 years old were identified from the Children's Oncology Group AREN03B2 study (July 2006-August 2016). Exclusion criteria were chemotherapy before pathologic evaluation or more than 30 days between imaging and surgery; in addition, patients with nephrogenic rests occurring within or juxtaposed to a Wilms tumor and patients with diffuse hyperplastic perilobar nephroblastomatosis were excluded. Two radiologists who were blinded to pathology results assessed all lesions. The two-sample test was used for continuous variables, and the Fisher exact test was used for categoric variables. ROC analysis was performed to determine the optimal size cutoff for distinguishing between nephrogenic rests and Wilms tumors. Thirty-one pathologically confirmed rests (20 perilobar, 11 intralobar) and 26 Wilms tumors smaller than 5 cm met the eligibility criteria for study inclusion. The median diameter of the nephrogenic rests was 1.3 cm (range, 0.7-3.4 cm) and the median diameter of the Wilms tumor was 3.2 cm (range, 1.8-4.9 cm) ( < 0.001). Imaging findings supportive of Wilms tumors were spherical ( < 0.001) and exophytic ( < 0.001) lesions. Perilobar rests (17/20) were more likely to be homogeneous than intralobar rests (3/11) or Wilms tumor (3/26) ( < 0.001). ROC analysis showed that the optimal size cutoff for distinguishing between nephrogenic rests and Wilms tumors was 1.75 cm. In children younger than 5 years old, the diagnosis of a Wilms tumor should be favored over a nephrogenic rest when a renal mass is spherical, exophytic, or larger than 1.75 cm. Homogeneity favors the diagnosis of perilobar nephrogenic rests, whereas intralobar rests and Wilms tumors are more likely to be inhomogeneous.