Is a 12cm complex cystic mass with thin septations and heterogeneous enhancement, located anterior to the right kidney in a 16-year-old patient with hematuria and normal kidney function, correctly described as an intrarenal mass?

Is This an Intrarenal Mass? Location and Differential Diagnosis

Mass Location and Terminology

No, this is NOT correctly described as an intrarenal mass—the imaging clearly indicates this is an extrarenal mass arising anterior to the right kidney, not within the kidney parenchyma itself. The report explicitly states "complex predominantly cystic mass...in the anterior aspect of the right kidney" with "both kidneys show normal sizes" and normal nephrograms bilaterally, indicating the kidneys themselves are uninvolved 1.

Key Imaging Features Supporting Extrarenal Origin:

• The mass is compressing and displacing the pancreas to the left, which would not occur with an intrarenal lesion 1
• Normal bilateral kidney function and appearance argues strongly against this being a kidney tumor 1
• The perinephric spaces are unremarkable, suggesting the mass is not arising from the renal parenchyma itself 1

Critical Differential Diagnoses in a 16-Year-Old

Most Likely Diagnoses (Extrarenal Origin):

1. Cystic Partially Differentiated Nephroblastoma (CPDN)

• This is the most concerning diagnosis given the patient's age, rapid growth (14cm in <1 year), and hematuria 2
• Multilocular cystic tumors with thin septations primarily affect boys during early childhood and adolescence 2
• CPDN contains embryonal cells in the septa and can recur locally, requiring complete surgical excision 2
• These lesions are characterized by multiple septations and heterogeneous enhancement patterns, matching this case 2

2. Cystic Nephroma

• Also presents as a multiloculated cystic mass with thin septations, grossly indistinguishable from CPDN 2
• More common in adult women but can occur in adolescent males 2
• Purely cystic with differentiated tissues, no blastemal elements 2
• Benign with excellent prognosis after surgical removal 2

3. Cystic Renal Cell Carcinoma

• Represents 5-7% of all renal tumors and can present as complex cystic masses 3
• In young adults (17-45 years), 79.8% of solid or complex cystic renal masses are malignant, with 95.8% being RCC 4
• However, young women are much more likely than men to have benign lesions (36% vs 9.5%), so gender matters here 4
• The heterogeneous enhancing portion at the inferior posterior region is concerning for malignancy 1, 5

4. Localized Cystic Disease of Kidney (LCDK)

• Rare, non-familial disorder presenting as multiple variable-sized cysts in one segment of the kidney 6
• Can cause hypertension in young adults through elevated renin levels 6
• Not progressive and not associated with renal function deterioration 6

Less Likely but Important Considerations:

5. Wilms Tumor with Cystic Degeneration

• Unusual at age 16 (typically younger children), but cyst formation can occur due to hemorrhage and necrosis 2
• Rapid growth pattern would be consistent 2

6. Cystic Clear Cell Sarcoma

• Rare but aggressive pediatric renal malignancy 2
• Would require different treatment approach than other cystic lesions 2

Immediate Management Algorithm

Step 1: Obtain Definitive Imaging

CT abdomen without and with IV contrast using a dedicated renal protocol is the gold standard for characterizing this mass 1, 5. The current imaging appears incomplete for definitive diagnosis.

• MRI without and with IV contrast is the preferred alternative, offering higher specificity (68.1% vs 27.7% for CT) for distinguishing benign from malignant masses 5, 7
• Apply Bosniak classification to the cystic components 5, 3
• The enhancing heterogeneous portion requires particular attention, as this suggests solid components (potential Bosniak IV) 5, 3

Step 2: Surgical Planning

Given the rapid growth (14cm in <1 year), hematuria, and heterogeneous enhancement, surgical excision is indicated regardless of final diagnosis 2, 4.

• Nephron-sparing surgery (partial nephrectomy) should be attempted if technically feasible, especially given the patient's young age and normal contralateral kidney 8, 4
• Complete surgical resection with negative margins is essential 8, 2
• Tissue marker placement may be considered if biopsy is attempted pre-operatively 1

Step 3: Consider Pre-operative Biopsy

Percutaneous biopsy of complex cystic lesions is well-tolerated and accurate in the majority of cases 3.

• However, biopsy is NOT recommended for purely cystic components due to low diagnostic yield 5
• Biopsy should target the solid enhancing portion if performed 5
• The concern about iatrogenic complications from percutaneous renal biopsy may be exaggerated in this patient population 3

Critical Pitfalls to Avoid

Pitfall #1: Assuming Benign Disease Based on Age

• While young women are more likely to have benign lesions, 79.8% of complex renal masses in young adults are malignant 4
• The rapid growth rate (14cm in <1 year) is a major red flag and strongly suggests aggressive pathology 1, 9

Pitfall #2: Inadequate Imaging Characterization

• Conventional ultrasound cannot reliably assess enhancement, which is essential for Bosniak classification 5, 7
• Small cysts (<1.5cm) are particularly challenging even with CT, but this 12cm mass should be well-characterized with proper imaging 5

Pitfall #3: Delayed Surgical Intervention

• Any mass with growth rate >5mm/year requires biopsy or excision to exclude malignancy 7, 9
• This mass grew 14cm in less than one year—immediate surgical consultation is mandatory 2, 4

Pitfall #4: Incomplete Surgical Resection

• CPDN can recur locally if incompletely excised 2
• Margin status must be negative 8

Additional Workup Considerations

• Check for von Hippel-Lindau syndrome or other familial RCC syndromes, as 12 of 114 young adults with renal masses in one series had familial disease 4
• Assess blood pressure, as LCDK can cause hypertension through elevated renin 6
• Serum creatinine and GFR should be documented pre-operatively 8
• Urinalysis to confirm hematuria and rule out infection 9