Arachnoid Cyst

In this pediatric patient, there is a large non-enhancing cystic lesion which follows CSF signal on all sequences and likely arises from the left middle cranial fossa with extension to the skull vertex resulting in marked mass effect and compression of a large portion of the left cerebral hemisphere and moderate rightward midline shift. There is no evidence of abnormal signal of the brain parenchyma showing that this is a slow growing/chronic process. Findings are compatible with a large arachnoid cyst which was treated with a shunt given the amount of mass effect and the potential for future enlargement. The middle cranial fossa is a common location.