There is a large lobulated cystic mass of the posterior cranial fossa (density similar to CSF) of retropontine location, centered on the 4th ventricle with prominent scalloped contours which is a characteristic feature. No solid component or peripheral enhancement seen. It shows downward extension filling the cisterna magna and foramen magnum. A mass effect is noted on the adjacent structures with mild dilatation of the supratentorial ventricular system (3rd and lateral ventricles).