NEURORADIOLOGY

Imaging features:
CT scan shows a well-defined hypodense mass with thick rim of calcification adjacent to the frontal horn of the left lateral ventricle. The attenuation of the mass lesion is consistent with fat (-20 to-40 HU). Similar fat density areas are found in the left sylvian fissure and left parietal sulci.

Imaging diagnosis:
CNS dermoid cyst rupture.

Discussion:
Dermoid tumors are not true neoplasms but inclusion cysts composed of ectodermal elements. They are uncommon lesions, accounting for approximately 0.3% of all brain tumors. They are often discussed with epidermoid tumors because of their similar appearances and developmental origins.

Dermoid and epidermoid tumors both contain stratified squamous epithelium found in skin, but have histologic differences. Epidermoid tumors are lined with stratified squamous epithelium and have an outer connective tissue capsule. Dermoid tumors have an outer connective tissue capsule and are lined with stratified squamous epithelium, which also contains hair follicles, sebaceous glands, and sweat glands. Centrally, both tumors contain desquamated epithelial keratin and some lipid material. The external surface of both tumors commonly has a smooth, lobulated, pearly appearance.
Common intracranial sites of dermoid tumors include the posterior fossa (within the fourth ventricle or cerebellar vermis) and suprasellar region.

Morbidity depends on the location of the tumor and on the involvement of adjacent structures. The rupture of a dermoid tumor can cause a granulomatous chemical meningitis that can lead to recurrent symptoms, most commonly headache. The subsequent meningeal inflammation may result in arterial vasospasm and, rarely, stroke and death.

On CT scans, it is typically a well-defined round hypodense mass. It typically has an attenuation consistent with fat (-20 to -40 hus). If there has been rupture, then scattered low density fatty droplets may be scattered throughout the ventricles and subarachnoid space. A fat/CFS fluid level may also be present. Calcifications, particularly in the capsule are particularly common. There is no enhancement after contrast administration.
MR imaging typically demonstrates high signal on T1 and variable signal on T2. This is consistent with the lipid and cholesterol which typically collects within the dermoid cyst. Our case is, in fact, atypical due to the low signal on T1. Hair may be noted as fine, low signal structures within the cyst. If the cyst ruptures, then high signal droplets on T1 images may be seen scattered throughout the CSF. Again, a fat/CSF fluid level may also be identified. As with other fatty masses, chemical shift artifact may also be present.
Angiographically, dermoid cysts typically show no significant enhancement. However, vasospasm and secondary infarction may be noted, particularly in cases with secondary meningitis from rupture of the cyst.

Contribution :
Dr Santosh Rai,
radiorai@rediffmail.com

Asst Professor, Dept of Radiodiagnosis, KMC Mangalore. Manipal Academy of higher education.
Consultant Radiologist, Global radiology center, WIPRO-MAHE, Bangalore.