Epidermoid cysts are slow-growing benign lesions resulting from the intradermal proliferation of epidermal cells. They are approximately twice as common in men as in women. Although they can present at all ages, they are particularly frequent in the 3rd and 4th decades of life. They are usually 1-4 cm in size. However, they can reach sizes of 15 cm or larger
2,3. While the cysts are generally asymptomatic, those that grow large and become infected or ruptured and those exhibiting malignant transformation are more likely to be symptomatic. Differential diagnosis includes abscess, cysts of the anal canal, pilonidal sinus or cyst, neurogenic tumor, lipoma, teratoma, perineal dermatitis, ganglion cyst, dermatofibrosarcoma protuberans, tailgut cyst, nodular fasciitis and myxoid tumor
3,5,6.
Although the etiology of epidermoid cysts is incompletely known, various theories have been proposed3. One hypothesis is that during embryogenesis, an aberrant settlement of ectodermal cells occurs during the cellular differentiation process. Another theory postulates a traumatic intradermal implantation of epidermal cells, such as an injection. This theory is frequently used to explain cysts on the extremities. Another common theory proposes a dermal cystic reaction to inflammation of pilosebaceous structures. This theory is typically used to explain the presence of cysts on the face, neck and trunk. Finally, a Human Papilloma Virus 60 infection of the eccrine canals is also proposed as a mediator of epidermoid cyst development. This theory is used to explain the lesions that appear only on the palms of the hands and soles of the feet3,7,8.
US and MRI are diagnostic means often employed to detect perineal lesions. US is useful for differentiating cystic and solid lesions. Epidermoid cysts may appear hypoechogenic, iso echogenic or hyperechogenic compared to surrounding structures on US examination. Uncomplicated cysts often have smooth walls and fail to show vascular flow signal. Nevertheless, vascular flow signals can be observed in the cyst wall of infected or inflammatory lesions. MRI is quite valuable in characterizing these lesions. Epidermoid cysts are usually hypointense on T1 weighted images and hyperintense in T2 weighted images. However, they may show different signal characteristics on both sequences. Uncomplicated lesions generally have smooth contours and do not show any enhancement or exhibit a thin peripheral enhancement after contrast agent administration. Ruptured lesions often appear septated and thick, irregular peripheral enhancement on postcontrast images due to the dermal inflammatory reaction caused by cyst contents, potentially generating an image similar to an abscess or a soft tissue tumor. Furthermore, a thick peripheral enhancement can be observed in infected/inflamed cysts and those with malignant degeneration1,3. In the literature, restricted diffusion was reported in one case on DW MRIs of a perineal epidermoid cyst1 as was our observation in the current case.
Definitive diagnosis of an epiermoid cyst necessitates histopathological examination. On histopathology, cysts are lined with stratified squamous epithelium that contains a granular layer and are filled with keratinous material that is often in a laminated arrangement. Their lipid architecture is similar to that of the epidermis. In addition, they express cytokeratin 1 and 10 which are constituent of the suprabasilar layers of the epidermis2,3. Although an epidermoid cyst is a frequently benign lesion, rarely malignant transformation can be observed. Squamous cell carcinoma, basal cell carcinoma or metastatic carcinoma may develop as a result of malignant transformation9. The reported frequency of squamous cell carcinoma arising from epidermal cysts ranges from 0.011 to 0.045%10.
As a result, the present study reports the DW MRI and US findings in a rare case of perineal epidermoid cyst. We believe that DW MRI might be useful for differential diagnosis of perineal lesions and thus a study can be conducted on this purpose. Moreover, we recommend employing DW MRI in cases where intravenous contrast agent is contraindicated.
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