Introduction
Dermoid cysts are benign congenital lesions that arise from ectodermal inclusions during neural tube closure or embryonic development. They are commonly seen in the head and neck region, especially the orbit, intracranial compartment, and ovaries.
Radiology is crucial for diagnosis, differentiation from other cystic lesions, and guiding management.
CT Imaging Features
- Well-defined, hypodense lesion with fat attenuation (−20 to −100 HU).
- May contain fat-fluid levels or calcifications (teeth, bone fragments).
- In intracranial dermoids: rupture can lead to fat droplets in the subarachnoid space or ventricles.
- Minimal or no contrast enhancement.
MRI Imaging Features (Preferred Modality)
Signal Characteristics
- T1-weighted: Hyperintense (due to fat content).
- T2-weighted: Variable signal; may be heterogeneous.
- Fat suppression sequences: Signal dropout confirms fat.
- FLAIR: Fat may remain hyperintense compared to CSF.
- DWI: No diffusion restriction (helps differentiate from epidermoid cyst).
Post-contrast
- Usually no enhancement; may have thin rim enhancement.
Key Radiology Signs
- Fat-fluid level → diagnostic clue.
- Ruptured intracranial dermoid → multiple T1 hyperintense droplets in ventricles and subarachnoid space (“chemical meningitis” may occur).
- Orbital dermoid → often superotemporal, well-circumscribed fat-density mass with bone remodeling.
Differential Diagnosis
- Epidermoid cyst (shows diffusion restriction, no fat).
- Lipoma (pure fat, no calcification or cystic component).
- Teratoma (more complex with soft tissue, fat, and calcification).
- Arachnoid cyst (follows CSF signal, no fat).
Role of Radiology
- Accurate diagnosis by detecting fat within the lesion.
- Pre-operative planning for surgical excision.
- Detection of complications (rupture, chemical meningitis).
Teaching Points
- Dermoid cyst = benign, fat-containing lesion.
- T1 hyperintensity + fat suppression dropout = diagnostic hallmark.
- Always differentiate from epidermoid cyst (DWI restriction).
- Look for signs of rupture in intracranial dermoids.
Conclusion
Dermoid cysts have distinctive imaging features on CT and MRI that allow confident diagnosis. Recognizing their fat content and differentiating them from mimics is essential for proper management and avoiding misdiagnosis.
