What is the most likely diagnosis for a young boy presenting with a large left-sided abdominal mass arising from the left kidney, distorting and displacing the collecting system, and multiple pulmonary nodules on chest X-ray?

Wilms Tumor is the Most Likely Diagnosis

In a young boy presenting with a large left-sided abdominal mass arising from the kidney that distorts and displaces the collecting system, combined with multiple pulmonary nodules on chest X-ray, Wilms tumor (Option A) is the most likely diagnosis. This clinical presentation represents metastatic Wilms tumor with pulmonary metastases, which is the classic pattern for this malignancy 1, 2, 3.

Key Diagnostic Features Supporting Wilms Tumor

Renal Origin with Collecting System Involvement

• Wilms tumor characteristically arises from the kidney parenchyma and distorts the collecting system, as demonstrated on IV pyelogram 1, 2, 4
• The mass effect on the collecting system is a hallmark finding that distinguishes Wilms tumor from other pediatric abdominal malignancies 4, 5
• Wilms tumor may present with abdominal mass, which is the most common presenting feature in children aged 2-5 years 1, 3, 6

Pulmonary Metastases Pattern

• Multiple pulmonary nodules represent the most common site of metastatic spread in Wilms tumor, occurring in approximately 10-15% of cases at presentation 1, 3
• The lung is the predominant site for hematogenous metastases in Wilms tumor, making this combination of findings highly specific 1, 6
• Chest X-ray showing multiple pulmonary nodules in the context of a renal mass strongly suggests metastatic Wilms tumor rather than other pediatric renal malignancies 3, 6

Why Other Diagnoses Are Less Likely

Neuroblastoma (Option B)

• Neuroblastoma typically arises from the adrenal gland or sympathetic chain, not from within the kidney itself 7, 6
• While neuroblastoma can displace the kidney, it does not typically distort the collecting system in the same manner as an intrarenal mass 7, 6
• Neuroblastoma more commonly presents with hypertension due to catecholamine excess or renal artery compression, which is not mentioned in this case 7
• Pulmonary metastases are less common in neuroblastoma compared to bone and bone marrow involvement 7, 6

Rhabdomyosarcoma (Option C)

• Rhabdomyosarcoma rarely presents as a primary renal mass 8, 6
• When occurring in the genitourinary tract, rhabdomyosarcoma typically involves the bladder, prostate, or vagina rather than the kidney 8, 6
• The pattern of arising from within the kidney and distorting the collecting system is not characteristic of rhabdomyosarcoma 6

Non-Hodgkin Lymphoma (Option D)

• While lymphoma can involve the kidneys, it typically presents as bilateral renal involvement or as part of systemic disease with lymphadenopathy 2, 6
• A large unilateral renal mass with collecting system distortion is not the typical presentation pattern for lymphoma 6
• Lymphoma more commonly presents with painless lymphadenopathy, fever, and weight loss rather than an isolated renal mass 2

Clinical Pearls and Diagnostic Approach

Immediate Workup Required

• Abdominal ultrasound is the optimal initial screening tool for pediatric abdominal masses due to lack of ionizing radiation and high sensitivity 2
• CT or MRI with contrast should follow for complete staging and characterization of the mass 2, 6
• Chest CT (preferred over chest X-ray) provides more accurate assessment of pulmonary metastases 2, 6

Important Considerations

• All pediatric patients with suspected malignant abdominal masses should be referred to specialized cancer centers with expertise in pediatric oncology 2
• Genetic testing should be considered, as certain syndromes (WT1 mutations, Beckwith-Wiedemann syndrome, DICER1 syndrome) predispose to Wilms tumor 1, 2
• Biopsy is generally not performed preoperatively for suspected Wilms tumor to avoid tumor spillage; diagnosis is typically confirmed at nephrectomy 6, 4

Common Pitfall to Avoid

• Do not delay referral to pediatric oncology for tissue diagnosis, as the combination of imaging findings (intrarenal mass with collecting system distortion plus pulmonary nodules) is sufficiently characteristic to proceed with staging and treatment planning 2, 6, 4