Nephrogenic rest (NR): persistent metanephric blastema ¹

• in the periphery: perilobular
• within the kidney: intralobular
• multiple NRs: nephroblastomatosis

NRs are important to diagnose and follow because they are precursors to Wilms tumor.

In some studies, up to 35% of diffuse hyperplastic perilobar NR transform to Wilms Tumor. This is considered the most at risk group 

The patient, in this case, demonstrates this in the initial ultrasound images.

Other syndromes associated with NRs (and Wilms tumor) are

• Beckwith-Wiedemann syndrome
• hemihypertrophy
• sporadic aniridia
• WAGR syndrome

On imaging, the NRs appear homogeneous on all imaging modalities. This is a very important distinction to Wilms tumor, which is heterogeneous. The NRs do not enhance to the same degree as the surrounding kidney

The patient, in this case, demonstrates homogeneous enhancement less that kidney parenchyma on the initial MRI images.

Unfortunately, a repeat MRI one year later demonstrates increased peripheral and internal enhancement, findings concerning for malignant transformation.