Case of the Month

Congenital Mesoblastic Nephroma (CMN):

Mesoblastic nephroma is the most common congenital renal neoplasm and patients typically present within the first three months of life. It is uncommon in children older than 1 year and comprises only 2% to 3% of pediatric renal neoplasms. In recent years, many CMNs have been detected during fetal sonography. Patients usually present with a unilateral mass in the renal sinus.
CNMs are generally categorized into three groups:

1. Classic CMN: The original descriptions of CMNs emphasized this pattern, which is actually present in a minority of cases (approximately 24% of cases). Grossly, it presents as a small mass with firm, whorled cut surfaces resembling a leiomyoma. Histologic findings resemble infantile fibromatosis (“fibromatosis-like” subtype): well-formed fascicles of spindle cells with bland nuclear features, abundant cytoplasm, and variable amounts of collagen deposition. It subtly infiltrates adjacent tissues and surrounds islands of native kidney at the interface between tumor and normal renal parenchyma. Long, narrow tongues of tumor typically extend into perirenal soft tissue, particularly in the renal hilum. Immunohistochemistry can show positive staining for SMA and renin, and occasionally WT-1, but negative staining for CD34, desmin, and keratin. Mitotic figures are usually rare and necrosis is generally absent. This subtype is not associated with any recurrent genetic abnormalities.
2. Cellular CMN: This is the most common subtype, comprising approximately 66% of CNM cases. Grossly, it presents as a large mass with foci of hemorrhage and necrosis. Histologic findings resemble infantile fibrosarcoma (“IFS-like” subtype): sheets or fascicles of densely packed plump spindle cells with high mitotic activity and a “pushing” border. Necrosis can be present. Cellular CMNs harbor the same genetic alteration as infantile fibrosarcoma: the t(12;15)(p13;q25) translocation which results in fusion of the ETV6 and NTRK3 genes.
3. Mixed CMN: This variant has features of both the cellular and classic variants and comprises approximately 10% of cases. Similar to the classic subtype, they do not have the ETV6 rearrangement.

Although both classic and cellular CMN have the capacity for aggressive local recurrence, only cellular CMN has metastatic potential (albeit minimal). Although cellular CMN histologically has a sarcomatous appearance  complete surgical excision is typically curative.

Differential Diagnosis:

• Wilms Tumor (nephroblastoma): Usually found in children older than 1 year of age, often bilateral. It has a triphasic histologic appearance rather than predominantly spindle cells.
• Rhabdoid tumor of the kidney (RTK): RTKs have more invasive margins, usually epithelioid cells with cytoplasmic inclusions and prominent nucleoli, and usually present with metastases. Loss of INI1 expression also is helpful in distinguishing RTKs from CMNs.
• Metanephric stromal tumor (MST): Usually occur in older patients (mean age of 2). On low power magnification they have nodular pattern. Heterologous differentiation and vascular changes can be seen in MSTs. Unlike CMNs, MSTs are positive for CD34 staining.
• Clear cell sarcoma of the kidney (CCSK): Plump cells separated by “chicken–wire” capillaries. Only 20% have clear cells. They have fine nuclear chromatin and low mitotic rate. CCSKs are negative for smooth muscle markers.