Epidermal inclusion cysts or epidermal cysts are common cutaneous lesions that represent proliferation of squamous epithelium within a confined space in the dermis or subdermis.
These are occasionally termed sebaceous cysts although, technically, this is a different entity. The term sebaceous cyst implies that the lesion originates in the sebaceous glands, which is not correct and, as such, the term epidermal inclusion cyst is preferred.
Epidermal cysts are either found incidentally or present as a firm non-tender lump. If they rupture a local inflammatory response to the necrotic debris released can mimic infection. Although they can be found anywhere, they are typically located on the scalp, face, neck, trunk, and back 1. Rarely they can be seen within bones representing an intraosseous epidermoid cyst 2.
Rarely epidermal cysts can undergo malignant degeneration with squamous cell carcinoma 1.
They are thought to occur as a result of 1,2:
- traumatic/surgical implantation
- occlusion of the pilosebaceous unit
- congenital rests of cells
- human papillomavirus type 57 or 60 infections implicated palmoplantar epidermoid cysts 1
On all modalities, they appear as well-circumscribed masses arising in or just deep to the skin.
Well-circumscribed predominantly hypoechoic mass. Typical shapes include 6:
- ovoid to spherical: ~70%
- lobulated: ~20%
- tubular: ~8%.
If small, it can mimic a typical anechoic cyst. Using subcutaneous fat as a reference, lesions tend to be hypoechoic. Larger lesions can be a little heterogeneous due to the presence of mucoid, fat, calcification or pus. Usually no associated vascularity. They, however, can have a variable appearance if ruptured, occasionally associated with vascularity and lobulated contours 6. As a useful tool to distinguish between an epidermal inclusion cyst and a dermoid cyst, if the lesion is not covered by periosteum, it is more likely to represent a dermoid cyst. On color Doppler, it may show twinkling artefact 8.
The density of epidermal inclusion cysts is similar to that of water. The margins are usually slender and sclerotic.
Imaging on MRI is similar to that of CNS epidermoid cysts or cholesteatomas, namely the content of the cyst is similar to CSF/water. In unruptured cases, typical signal characteristics include:
- T1: low/intermediate signal
- T2: high signal
T1 C+ (Gd)
- no enhancement centrally
- may have thin peripheral enhancement
With ruptured cases, they may have septa, show thick and irregular rim enhancement, and can be accompanied by a fuzzy enhancement in surrounding subcutaneous tissues 1.
Treatment and prognosis
They are benign and generally do not require treatment. If infected they may require incision and drainage. If they continue to grow they may require excision.
- superimposed infection
- concurrent occurrence of tumors within them, e.g. melanoma (very rare) 4
General imaging differential considerations include:
- ganglion cyst (if near a joint)
- neurogenic tumors e.g. neurofibroma
- nodular fasciitis
- myxoid tumors
- dermatofibrosarcoma protuberans
- 1. Hong SH, Chung HW, Choi JY et-al. MRI findings of subcutaneous epidermal cysts: emphasis on the presence of rupture. AJR Am J Roentgenol. 2006;186 (4): 961-6. doi:10.2214/AJR.05.0044 - Pubmed citation
- 2. Baek HJ, Lee SJ, Cho KH et-al. Subungual tumors: clinicopathologic correlation with US and MR imaging findings. Radiographics. 2010;30 (6): 1621-36. doi:10.1148/rg.306105514 - Pubmed citation
- 3. Sonographic Findings of Ruptured Epidermal Inclusion Cysts in Superficial Soft Tissue Emphasis on Shapes, Pericystic Changes, and Pericystic Vascularity Wook Jin, MD, Kyung Nam Ryu, MD, Gou Young Kim, MD, Hyun Cheol Kim, MD, Jae Hoon Lee, MD, Ji Seon Park, MD. J Ultrasound Med 2008; 27:171–176
- 4. Bajoghli A, Agarwal S, Goldberg L et-al. Melanoma arising from an epidermal inclusion cyst. J. Am. Acad. Dermatol. 2013;68 (1): e6-7. doi:10.1016/j.jaad.2012.04.010 - Pubmed citation
- 5. Jin W, Ryu KN, Kim GY, Kim HC, Lee JH, Park JS. Sonographic findings of ruptured epidermal inclusion cysts in superficial soft tissue: emphasis on shapes, pericystic changes, and pericystic vascularity. Journal of ultrasound in medicine : official journal of the American Institute of Ultrasound in Medicine. 27 (2): 171-6; quiz 177-8. Pubmed
- 6. Lee HS, Joo KB, Song HT, Kim YS, Park DW, Park CK, Lee WM, Park YW, Koo JH, Song SY. Relationship between sonographic and pathologic findings in epidermal inclusion cysts. Journal of clinical ultrasound : JCU. 29 (7): 374-83. Pubmed
- 7. Bansal AG, Oudsema R, Masseaux JA, Rosenberg HK. US of Pediatric Superficial Masses of the Head and Neck. (2018) Radiographics : a review publication of the Radiological Society of North America, Inc. 38 (4): 1239-1263. doi:10.1148/rg.2018170165 - Pubmed
- 8. Clarke R, Suresh P, Thomas R, Freeman S. Twinkle artefact in the ultrasound diagnosis of superficial epidermoid cysts. Ultrasound. 2016 Aug;24(3):147-153. Epub 2016 Jul 12. Pubmed