CASE 37: 16 m/o male patient with a palpable mass

Here is a case in which Radiology plays an important role in management. This case was provided by Tecniscan Diagnostic Center.

This was a 16 month male patient with intermitent constipation with a palpable pelvic mass that initially was thought to be a dilated sigmoid but later pediatrician noticed that the mass had smooth edges and had a circular configuration. An ultrasound was made (not shown) that reported distended bowel loops hence difficulted the correct visualization, although a pelvic heterogeneous mass was seen mostly echoic with acoustic shadowing but barely depicted. Padiatrician recommended a CT scan, here are the images:

Findings and differentials??...later on I'll post the conclusion of this case.