Congenital mesoblastic nephroma associated with polyhydramnios and hypercalcemia

Congenital mesoblastic nephroma (CMN) can present with atypical clinical and imaging findings. A premature male neonate was born to an 18-year-old woman after 33 weeks' gestation, which was complicated by polyhydramnios and placenta abruptio. A tight abdominal mass was diagnosed antenatally. From the 1st day of life, the newborn had hypercalcemia with initially normal parathormone levels and polyuria for the first hours of life and normal urine output afterwards. Ultrasonographic study and magnetic resonance imaging of the abdomen showed at the upper pole of the right kidney a heterogeneous, solid, poorly defined mass, partially surrounded by a subcapsular fluid collection mimicking malignant rhabdoid tumor of the kidney. Surgical resection revealed a CMN of mixed, classic, and in areas, cellular type. One year after the resection, the patient is asymptomatic and normocalcemic. In conclusion, CMN may present with atypical clinical and imaging findings, necessitating an extensive work-up in order to exclude highly malignant renal tumors of the neonatal period.