Abdomen / Pelvis for Wednesday, April 9th, 2025

History

4-month-old female with antenatal diagnosis of bilateral renal enlargement and hydronephrosis.

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Question

Which of the following imaging features is most suggestive of Wilms tumor rather than nephrogenic rests?

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Correct answer

Spherical shape

Discussion

Nephrogenic rests are clusters of embryonal renal cells that persist beyond normal nephrogenesis, typically past 36 weeks of gestation. While these lesions may remain dormant or undergo sclerosis, they also pose a risk of neoplastic transformation, particularly into Wilms tumor. Notably, nephrogenic rests are found in approximately 28–40% of unilateral Wilms tumors and nearly all cases of bilateral Wilms tumors. Their classification into perilobar(along the periphery of the renal lobe) and intralobar (within the renal lobe) types is significant, as these variants differ in location, cellular composition, and associated syndromic risk.  

Distinguishing nephrogenic rests from Wilms tumor is challenging based on imaging and even needle biopsy. Pathological differentiation often relies on assessing the lesion’s interface with adjacent renal parenchyma. Perilobar nephrogenic rests demonstrate an abrupt interface with adjacent kidney tissue, lacking a pseudo capsule, whereas Wilms tumors exhibit a well-defined pseudo capsule. In contrast, intralobar nephrogenic rests intermingle with normal renal parenchyma at the periphery, distinguishing them from the more encapsulated Wilms tumor. Moreover, perilobar nephrogenic rests tend to be more homogeneous in texture, often exhibiting an oblong rather than spherical morphology—features that aid in imaging differentiation but are not universally reliable.  

Imaging studies suggest that nephrogenic rests are typically smaller than Wilms tumors, with a general cutoff of 2 cm, though they can reach up to 5 cm. Homogeneity and an elliptical shape have been suggested as primary imaging characteristics favoring nephrogenic rests. The AREN03B2 trial, an ongoing study by the Children’s Oncology Group, has sought to refine imaging criteria to better distinguish nephrogenic rests from Wilms tumor. According to their report, there is an overlap in imaging findings between nephrogenic rests, particularly intralobarnephrogenic rests and Wilms tumor. They suggested a size cut-off of 1.75 cm to distinguish between nephrogenic rests and Wilms tumor. Lesion margin, indistinct versus sharp; and lesion characteristics, homogenous versus heterogenous, were not helpful to discriminate between nephrogenic rests and Wilms tumor. Their results supported that a spherical shape is more likely associated with Wilms tumor. In addition, they reported that if a peripheral lesion bulges beyond the renal contour, it is more likely to be a Wilms tumor. 

In the index case, the presence of a large right-sided intralobar lesion with diffusion restriction raised concern for early neoplastic transformation. The histopathologic findings of blastemal, epithelial, and possible stromal components are characteristic of Wilms tumor but did not demonstrate anaplasia. Given that hyperplastic nephrogenic rests may closely mimic small Wilms tumors, a comprehensive evaluation integrating imaging features, biopsy results, and lesion growth patterns is essential for optimal management.  

Considering the risks of malignant transformation, patients with multiple or hyperplastic nephrogenic rests require close surveillance. MRI, particularly with diffusion-weighted imaging, is a valuable tool for monitoring these lesions over time. Increased diffusion restriction and rapid interval growth may indicate progression to Wilms tumor, warranting intervention. Management should be tailored to the individual, balancing the risk of malignancy to preserve renal function, especially in cases with bilateral involvement.  

This case highlights the complexities of diagnosing and managing nephrogenic rests, emphasizing the importance of multimodality assessment. 

Differential diagnosis

Wilms tumor.

Nephrogenic rests. 

References

• 1. Lowe LH, Isuani BH, Heller RM, et al. Pediatric renal masses: Wilms tumor and beyond. *Radiographics.* 2000;20(6):1585-1603. 2. Faria P, Beckwith JB, Mishra K, et al. Nephrogenic rests, nephroblastomatosis, and the risk of Wilms tumor. *Pediatr Dev Pathol.* 1999;2(4):279-289. 3. Khanna G, Rosen N, Anderson JR, et al. Diagnostic imaging of pediatric renal tumors: Wilms tumor and beyond. *Radiol Clin North Am.* 2011;49(4):689-706. 4. Shamberger RC, Haase GM, Perlman EJ, et al. Imaging Characteristics of Nephrogenic Rests Versus Small Wilms Tumors: A Report From the Children’s Oncology Group Study AREN03B2. *Ann Surg Oncol*. 2020;27(6):1793-1801. doi:10.1245/s10434-019-08089-2.