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Background
Wilms tumor (WT) is the most common renal tumor in children. The risk of developing WT varies by ethnicity, with Black patients at highest risk and Asian patients at the lowest risk. WT typically presents as unilateral abdominal fullness. Imaging is important to confirm the diagnosis, determine staging, and follow for complications.

Educational Goals / Teaching Points
Viewers of this education exhibit will become familiar with typical and atypical imaging findings for WT, be reminded of associated complications, understand the role of imaging in determining treatment approach to WT, and be able to provide follow-up imaging recommendations after initial therapy.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Imaging typically demonstrates a solid, heterogeneous renal mass that commonly displaces adjacent structures. First-line imaging is most commonly ultrasound. Classically, the claw sign is visualized where sharp angles are formed between the mass and the native renal parenchyma. On CT and MRI, WT appears as a large, heterogeneous renal mass with poor enhancement. The claw sign is also appreciated on CT and MRI, with the latter as the modality of choice used for staging. WT staging is determined by anatomic extent, independent of genetic risk factors and histological findings. Radiologists must be aware of key staging characteristics including containment within the renal capsule without vascular involvement (stage I), extent beyond the renal capsule (stage II), spread to regional lymph nodes (stage III), distant spread (stage IV), and bilateral renal involvement (stage V). With initial imaging, chest CT is necessary to evaluate for pulmonary metastases to help stage and guide therapy, as the lungs are the most common site for WT to spread. Treatment consists of chemotherapy and surgical resection, the order of which is typically determined by the stage and treatment protocol being used. Stage I and II tumors can typically be removed in their entirety, whereas larger or more advanced tumors may require down-staging with chemotherapy prior to surgery. Tumor characteristics used for staging provide necessary information to assist in surgical planning and reduce the risk of intra and postoperative complications. Complications of WT include invasion to adjacent structures, metastasis, and recurrence. Follow-up imaging recommendations include yearly abdominal ultrasounds to screen for recurrence or new foci of disease. For children with pulmonary metastases, a chest CT at 6 weeks posttherapy is also recommended to evaluate treatment response. Also, FDG PET CT is another imaging modality commonly used for assessment both before and following treatment.

Conclusion
It is important for all radiologists to be aware of WT imaging characteristics in typical and atypical presentations. Early detection with thorough tumor assessment is key, as earlier stage at time of diagnosis corresponds with a more favorable prognosis. Appropriate knowledge of treatment protocols and follow-up imaging requirements is important to mitigate the consequences of complications such as metastasis and recurrence.