You will be given a definition of a task first, then some input of the task.
Given an abstract, generate a keyword (a noun phrase) that best describes the focus or contribution of the paper. Such keywords can be directly from the given abstract or outside it.

Abstract: The authors describe four cases of subependymoma studied with computed tomography (CT) and review 18 previously reported cases in an attempt to define the most characteristic CT presentation of this rare, benign tumor. Subependymoma usually appears as an isodense, or even hypodense, intraventricular tumor on plain CT scan and shows minimal or no enhancement in postcontrast studies. Differential diagnosis between subependymoma and the more malignant true ependymoma is difficult, particularly when the tumor occurs in the posterior fossa. Recognition of subependymoma should prompt the surgeon to attempt radical tumor removal because it can be achieved without sacrificing contiguous tissue and carries a good prognosis.
Output:
Adolescence