Hypodense colloid cyst of the third ventricle.

The diagnosis of colloid cysts of the third ventricle using computed tomography (CT) rarely causes problems. The typical well-rounded, hyperdense mass in the anterior part of the third ventricle is usually well shown, along with the associated lateral ventricular dilatation. However, it is also well recognized that these cysts may be isodense, and this can on occasion lead to diagnostic difficulty. Rarely have hypodense colloid cysts been reported. We describe two patients in whom the low density of the tumors led, initially, to a failure to identify them on the CT scan. Although this is an atypical appearance, it is important to be aware of it, as colloid cysts which are surgically curable have a definite morbidity and mortality if untreated.