So, what is an epidermoid cyst? It is a very common skin ailment. However, when this type of cyst occurs on the breast, it is usually cause for a breast cancer screening mammogram.
So an epidermoid cyst is a small, palpable bump on the skin usually in a ‘dome’ shape and appearing as a sore. Epidermoid cysts are always unilocular (occur as ‘one thing‘) and have a characteristic ‘pearl-like’ sheen. Sometimes doctors will informally refer to an epidermoid cyst as a ‘pearly tumor’, but they are not tumors.
There is a newer page with more up-to-date information on all types of breast cysts including epidermoid cysts.
Epidermoid cysts are completely benign when they occur on the breast, but can be a serious concern if they occur near the brain or central nervous system (CNS).
Squamous cell carcinoma may possibly arise in an epidermoid cyst, but this situation is exceptionally rare, and completely unrelated to breast cancer. Most epidermoid cysts form on the face, neck and trunk.
An epidermoid cyst develops out of ectodermal tissue. A thin layer of squamous epithelium makes up an epidermoid cyst. The interior of an epidermoid cyst is filled with ‘keratohyalin‘ (Keratin) from the ongoing ‘desquamation’ of the epithelial cell lining.
(Desquamation refers to the shedding of the outer layers of the skin, like ‘taking the scales off of a fish’, except here it is happening in the inner layers of the skin).
An epidermoid cyst is a welcome diagnosis
For a woman worrying about a breast lump, the discovery of an epidermoid cyst is a welcome one. However, it is really not possible to find this out definitively without a biopsy, which can be a stressful process, quite understandably.
Causes of epidermoid cysts
No one is exactly sure what causes an epidermoid cyst. Epidermoid cysts are likely the result of a proliferation of epidermal cells within a circumscribed space of the dermis.
Scientists believe that these types of cysts are a genetic anomaly that arise from small nests of cells ‘left over’ from an embryonic phase of development (while you were still in the womb).
In spite of their ‘developmental’ origin, epidermoid cysts are rare before the third decade of life.
Trauma or Surgery can cause Epidermoid Breast Cysts
The ‘acquired’ type of epidermoid cysts is the result of trauma or some surgical event, such as:-
- a core needle biopsy
- partial mastectomy
This somehow stimulates epithelial proliferation or squamous cell metaplasia. Sometimes these cysts can be an indirect result of body piercings.
Diagnostic imaging of Epidermoid Breast Cysts
A breast cancer screening mammogram typically follows the discovery of a likely epidermoid cyst.
These type of cysts are sometimes similar in appearance to fibroadenomas and complex cysts, but also to medullary or mucinous carcinomas. For this reason, medics need to evaluate epidermoid cysts quite seriously. When the cyst is very large, the possibility of a phylloid breast tumor cannot be ruled out.
Well-circumscribed benign mass
Radiologists usually describe epidermoid cysts as benign masses with well-circumscribed margins.
Ultrasound usually reveals epidermoid cysts to be solid, but there can be calcifications within an epidermoid cyst. These microcalcifications will present as internal echogenic foci on a sonogram.
With a CT scan an epidermoid cysts will appear as a lucent lobulated mass.
An MRI usually reveals an epidermoid cyst to be an ‘isointense‘ lesion (one with the same density as shown on other imaging studies), but a definitive diagnosis often requires an excisional biopsy.
It is virtually impossible to confirm am epidermoid cyst without a biopsy
The mammographic x-ray below shows a breast lesion with a round shape and smooth margins. However, it is impossible to tell whether or not the contents of the cyst are liquid or solid.
A biopsy will likely be necessary.
Microcalcifications, Epidermoid Cysts and Mammograms
If an epidermoid cyst in the breast contains calcifications, a breast ultrasound will likely detect them.
The sonograph of an epidermoid cyst below does not appear to contain calcifications, however. A radiologist is able to determine the image below is most likely not cancer, but can not say exactly what it is.
The lesion is dark enough inside the round shape to indicate that is is probably not a tumor made of living cells. At the same time, it is not purely black, which indicates pure fluid. So, we know that it is something thicker than liquid.
This ultrasound therefore looks benign. However, an ultrasound can not distinguish whether the cyst is filled with a ‘flaky’ and ‘dandruffy’ type of substance, (kind of a watery paste). Or perhaps some other event that has hemorrhaged into a cyst.
Some Calcifications/microcalcifications within epidermoid cysts
So, if microcalcifications appear on a screening mammogram an excisional or core biopsy is necessary. However, the chances of finding something worrisome are very slight.
Histological characteristics of breast epidermoid cysts
Overall, an epidermoid cyst appears as a ‘well-defined’ lobulated cystic lesion. That is many lobules make up the cyst and it also has clear borders and high density.
These cysts have a lining of thin, stratified layers of squamous epithelium. Furthermore, these cysts are full of keratin, which usually appears as a thick, flaky, (scaly) white material.
Keratin deposits look like ‘flakes‘ on a breast X-ray
On the cytologic image of a breast epidermoid cyst below, keratin deposits look like ‘tree bark’. However, on mammogram keratin looks like rust flakes or scales.
The ‘scaling’ or ‘flaking’ that comprises much of the solid mass of an epidermoid cyst are clearly evident in the next image below.
Malignant breast cancer cells would not look anything like this, yet on a mammogram, there could be similarities.
Other confusing terms for Epidermoid Cyst:
Epidermoid cysts is a term that, in the past, medics frequently used to describe sebaceous cysts as well.
‘Epidermoid’ is the more all inclusive term, and they are very similar but not exactly the same. On a clinical exam, it is virtually impossible to distinguish sebaceous cysts from epidermoid cysts.
Breast sebaceous cysts are foul smelling
Sebaceous cysts originate in the infundibular portion of the hair follicles. Sebaceous cysts also have a very foul odor. Specifically, however, the ‘contents’ of a sebaceous cyst are different from epidermoid cysts. Sebaceous cysts are filled with a clear, oily liquid made by sebaceous (grease) glands.
Epidermoid cysts can also contain some amounts of the white, flaky, semi-solid, Keratin-like discharge found in epidermoid cysts. These cysts may also contain a mixture of oily and white-flaky junk that medics call ‘sebum’.
But in modern usage, the term sebaceous cyst is more specifically for cysts containing primarily the oily discharge of the sebaceous glands.
Breast cysts with infection are harder to differentiate radiologically
Inflammatory fold, axillary and medial areas are common locations for sebaceous cysts.
If an infection has caused complications to the cyst the margins of these masses may become ‘shaggy’ and irregular with thickening of the overlying skin.
On a mammogram, it becomes more difficult to differentiate the cysts from neoplastic cell growth.
Pilar breast cysts are associated with hair cells, not skin cells
The same all-inclusive term epidermoid cyst also applies to the ‘Pilar’ cyst, another sub-type.
Pilar cysts largely contain keratohyalin and not sebaceous-gland oils. So, the cyst ‘lining’ is different than with epidermoid cysts because hair root cells, instead of skin cells, are the main content.
- Chang Tsun-Hou Hsu Ciu-Cheng, Hsu Hsian-He. (et al) .(2004) Epidermoid Cyst of the Breast Presenting as a Mass with Internal Calcifications and so Mimicking Carcinoma. J Med Sci 2004;24(6):325-328
- Jehle KS, Shakir AJ, Sayegh ME. (2007) Squamous cell carcinoma arising in an epidermoid cyst. British journal of hospital medicine (London, England : 2005) 68 (8): 446 https://www.ncbi.nlm.nih.gov/pubmed/17847698
- Lopez-Rios F, Rodriguez-Peralto JL, Castano E, Benito A. (1999) Squamous cell carcinoma arising in a cutaneous epidermal cyst: case report and literature review. Am J Dermatopathol 1999;21:174-177. https://www.ncbi.nlm.nih.gov/pubmed/10218680
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- Morris PC, Cawson JN, Balasubramaniam GS. (1999) Epidermal cyst of the breast: detection in a screening programme. Australas Radiol 1999;43:12-15. https://www.ncbi.nlm.nih.gov/pubmed/10901863
- Zuber TJ. (2002) Minimal excision technique for epidermoid (sebaceous) cysts. Am Fam Physician 65 (7): 1409–12, 1417–8, 1420. https://www.ncbi.nlm.nih.gov/pubmed/11996426