Nephroblastomatosis: imaging challenges

Nephroblastomatosis is a rare pre-neoplastic lesion defined as the presence of multiple nephrogenic rests. These are abnormal foci of persistent embryonal cells beyond 36 weeks’ gestation that are thought to be precursors for the development of Wilms tumor. Nephrogenic rests are seen in approximately 1% of pediatric autopsy series. Nephrogenic rests and nephroblastomatosis represent a spectrum of the same disease and the distinction between the two is somewhat arbitrary. There are no formal quantitative criteria that determine how many nephrogenic rests constitute nephroblastomatosis. The nephrogenic rests are divided into four categories in the pathology literature: perilobar, intralobar, combined and diffuse, depending on their location within the renal parenchyma. On imaging, nephroblastomatosis is best classified as multifocal or diffuse. Virtually all cases of bilateral Wilms tumor have associated multifocal nephroblastomatosis. Advances in imaging have increased our ability to detect small renal lesions in situ and, therefore, have played an increasingly significant role in the preoperative diagnosis and postoperative management of nephroblastomatosis and Wilms tumor. Imaging studies are essential in the diagnosis of these lesions and their evaluation over time to detect transformation into Wilms tumor. State-ofthe-art imaging with contrast-enhanced CT and MRI routinely detects lesions as small as 5 mm in diameter. Nephroblastomatosis is considered a predisposing condition to Wilms tumor development both in the ipsilateral and contralateral kidney. Identifying and distinguishing between nephroblastomatosis and Wilms tumor is critical because they imply distinct prognoses and treatment decisions. Differentiation between Wilms tumor and nephroblastomatosis is based on histological evaluation of biopsy combined with imaging studies. Fine-needle aspiration cytology is of limited value in the assessment of these lesions, as cytology is not able to distinguish nephroblastomatosis from Wilms tumor. The anatomical location of nephrogenic rests varies as well as the pattern of their distribution. Importantly, multifocal nephroblastomatosis has a stronger association with Wilms tumor than the diffuse type. The diffusely hyperplastic perilobar nephroblastomatosis has a characteristic appearance that is identified as peripheral confluent masses distorting and compressing the kidney with a thick rind of uniform abnormal tissue. On CT and MR imaging, both types appear similar to the renal cortex on nonenhanced scans. After contrast administration, NRs become markedly hypodense on CT and hypointense on MRI in comparison with the intensely enhancing renal parenchyma. Although there are no absolute imaging criteria to differentiate nephrogenic rests and nephroblastomatosis from Wilms tumor, nephrogenic rests and nephroblastomatosis can be differentiated from Wilms tumor in the same kidney, as Wilms tumors tend to be larger and more heterogeneous and demonstrate some contrast enhancement. Gadolinium-enhanced T1-weighted MRI is currently touted as the optimal imaging approach for detecting nephrogenic tissue, although US and CT are commonly Disclaimer Dr. Grattan-Smith has no financial interests, investigational or off-label uses to disclose.