Mesoblastic Nephroma

Mesoblastic nephroma (also known as fetal renal hamartoma or leiomyomatous hamartoma) is the most common neonatal renal tumor and the most frequent benign renal tumor in childhood. Up to 10% of all pediatric renal tumors are mesoblastic nephromas. While classified as benign, local invasion and hematogenous metastases may occur. Despite this tendency, the renal pelvis and vascular pedicle are spared. Additional complications have to do with mass effect and hormonal secretions that may result in polyuria, polyhydramnios in utero, gastrointestinal obstruction, and hypercalcemia.

Two types, the classic variant and the cellular variant, have been described. The cellular type tends to have a poorer prognosis and tends to affect children older than 3 months.

On ultrasound, we'll see a homogeneous echogenic mass arising from the kidney. The classic variant lesions may have a peripheral, hypoechoic, vascular ring on ultrasound or a large solid component, while cellular variant lesions tend to have cystic/necrotic change and central hemorrhage. The vascular ring is similar to those seen in thyroid lesions and represents dilated blood vessels at the periphery of the mass.

On CT, they present as a solid, nonenhancing renal mass with cystic or hemorrhagic components. No calcifications are seen. The collecting system may be enclosed by the mass, leading to apparent contrast excretion within the mass. The renal pelvis, however, is spared. Invasion of the vascular pedicle is also not seen.

T1-weighted images show a lesion with signal intensity characteristics similar to renal cortex and skeletal muscle. T1-weighted images show a hyperintense lesion. Minimal or no contrast-enhancement is seen within the mass, but classic variant lesions may have a peripheral, markedly enhancing ring that may correspond to the vascular ring seen on ultrasound.

The main differential consideration is congenital Wilms tumor. Unfortunately, imaging cannot differentiate the two and biopsy is required. Congenital adrenal neuroblastoma and multicystic dysplastic kidney may also be considered based on imaging findings.

References

• Lowe LH, Isuani BH, Heller RM, Stein SM, Johnson JE, Navarro OM, Hernanz-Schulman M. Pediatric renal masses: Wilms tumor and beyond. Radiographics. 2000 Nov-Dec;20(6):1585-603.
• Rieumont MJ, Whitman GJ. Mesoblastic nephroma. AJR Am J Roentgenol. 1994 Jan;162(1):76.
• Singh SP. Mesoblastic Nephroma. eMedicine. Oct 27, 2009.