Epidermoid cysts probably occur from inflammation of pilosebaceous structures. The second theory is that the formation of an epidermoid cyst is related to the implantation of epidermis into the dermis through trauma (example: intramuscular injection) and migration during embryogenesis. A later theory frequently valid is for intracranial lesions3
. The epidermoid cyst of our case is located on the forearm but the patient had no trauma or injection history. Lee et al4
showed the sonographic findings of epidermoid cysts, which can have lobulated contours and show color Doppler signals, mimicking a solid mass. Similar to his study we showed that the mass was not anechoic as a cyst, instead it was hypoechoic, heterogeneous and showed color Doppler signals as solid masses. Unfortunately, we did not record the ultrasonographicimages of our patient.
Hong et al reported that MRI findings of unruptured epidermal cysts were hypointense on T1 and hyperintense on T2-weighted images relative to the muscle5. On postcontrast T1- weighted images, peripheral rim enhancement was seen. Epidermoid cysts may have hyperintense regions on T1-weighted images compared to muscle6, and may be hypointense on T1 and T2-weighted images related to dense debris and calcification components7. Shibata et al reported that no enhancement was observed inside the tumors and the variety of signal intensities on T1 and T2-weighted images reflects differences in chemical components of the epidermoid cysts2.
Our case was hypointense on T1-weighted images, hyperintense on T2-weighted images compared to muscle. This case had a variable amount of serpiginous lower signal foci on T2- weighted images. We thought that heterogeneity on T2-weighted images related to the cyst lumen was filled with keratin arranged in laminated layers.
The differential diagnosis of an epidermal cyst contains a fibrous tissue tumor, such as benign fibroma, xanthoma, malignant fibrous histiocytoma or fibrosarcoma, other subcutaneous cystic masses, vascular lesions, cystic degeneration and hemorrhage, lymphangioma with hemorrhage, cystic teratoma, large ganglion cyst and echinococcal cyst5,6 Ganglion cysts are hyperintense on T2-weighted images, and show peripheral rim enhancement on postcontrast images. The heterogeneous signals of epidermoid cysts on T2-weighted images may help differentiation of epidermal cysts from other fluid cysts (ganglion cyst, bursitis). Some solid tumors such as neurogenic tumors, nodular fasciitis, myxoid tumors may be hyperintense on T2-weighted images, so they mimic cystic masses. The enhancement pattern of these solid masses can be used to differentiate epidermal cysts5.
In conclusion, an epidermal cyst should be thought in the differential diagnosis of a cystic soft tissue masses. Useful features for the diagnosis of an epidermoid cyst of the extremity are a well-defined border, round or oval lesion, subcutaneous location, hypointense on T1- weighted images, hypointense foci in the hyperintense background on T2- weighted images, and no enhancement on postcontrast images.