Orbital Dermoid/Epidermoid

Dermoids of the orbital region can be classified as superficial subcutaneous (also known as simple or exophytic) or deep (also known as complicated or endophytic). Superficial dermoids present in infancy and are not associated with proptosis. Deep dermoids are usually seen in adults and are associated with proptosis. They can cause scalloping of adjacent bone and even bone defects, allowing them to extend into adjacent paranasal sinuses, into the temporal fossa, intracnanially, or through the orbital fissures.

Radiographs and CT may show a soft tissue mass causing scalloping of bone. CT may also show the fat content, although the attenuation may be higher in mixed lesions. MRI is the best modality for demonstrating intracranial extension. MRI findings are variable. T1-weighted images will typically show a strongly hyperintense lesion if fat predominates, but will be intermediate intensity otherwise. On T2-weighted images, the lesions are typically isointense or mildly hypointense. Post-contrast images reveal thin rim-enhancement.

The image above shows a case of pathologically proven periorbital dermoid. The MRI characteristics aren't those typically described for these lesions. There is a well-defined cystic lesion within the subcutaneous tissue of the left lateral orbital wall adjacent to the left frontozygomatic suture. No intracranial or intraorbital extension was seen, but there was bony remodeling. The lesion has a thin rim that demonstrates minimal enhancement. DWI and ADC images didn't reveal restricted diffusion.

References

Nugent RA, Lapointe JS, Rootman J, Robertson WD, Graeb DA. Orbital dermoids: features on CT. Radiology. 1987 Nov;165(2):475-8.