An epidermoid cyst is a common skin ailment, but when it occurs on the breast it is usually cause for a breast cancer screening mammogram. It is a small, palpable bump on the skin, usually in a ‘dome‘ shape and appearing as like a sore. Epidermoid cysts are always unilocular (occur as ‘one thing‘) and have a characteristic ‘pearl-like‘ sheen (sometimes an epidermoid cyst is informally referred to as ‘pearly tumor‘, but they are not tumors).
So this page is getting somewhat a little bit ‘out-dated’ (but you can still use it, as it still has great material), so I have came to the decision to create a newer version with more up-to-date information on Epidermoid Cysts of the Breast.
They are completely benign when they occur on the breast, but can be a serious concern if they are found near brain and central nervous system (CNS) elements. 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, and is made of a thin layer of squamous epithelium. The interior of an epidermoid cyst is filled with ‘keratohyalin‘ (Keratin) from continued ‘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 concerned about a breast lump, a discovery of an epidermoid cyst is a welcome one, but 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. It is thought to be a genetic anomaly that arises 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.
Some epidermoid breast cysts are caused by trauma or surgery
The ‘acquired‘ type of epidermoid cysts is the result of trauma or some surgical event, such as a core needle biopsy, lumpectomy, or partial mastectomy, which 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. They are sometimes similar in appearance to fibroadenomas and complex cysts, but also to medullary or mucinous carcinomas, so they have to be evaluated 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, which 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 as 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. But it is impossible to tell whether or not the contents are liquid or solid.
A biopsy will likely be required.
Microcalcification would be picked up on a mammogram
If an epidermoid cyst in the breast contained calcifications, they would likely be detected on a breast ultrasound. The sonograph of an epidermoid cyst below does not appear to contain calcifications, however. A radiologist would be able to determine that it is most likely not cancer, but could not say exactly what it is.
The lesion is dark enough inside the rounded shape to indicate that is is probably not a tumor made of living cells. At the same time, it is not purely black, which would indicate pure fluid, so it is something thicker than liquid.
This ultrasound therefore looks benign, but ultrasound can’t distinguish whether its filled with a ‘flaky‘ and ‘dandruffy‘ epidermoid cyst, (kind of a watery paste) or perhaps some other event which has hemorrhaged into a cyst.
Some Calcifications/microcalcifications within epidermoid cysts
Between 10-25% of epidermoid cysts are shown to have calcification. As microcalcifications are a strong indicator of potential Ductal Carcinoma in situ (DCIS), their appearance on a screening mammogram does warrant an excisional or core biopsy, even though the chances of finding something worrisome are very slight.
Histological characteristics of breast epidermoid cysts
Overall, an epidermoid cysts appears as a ‘well-defined‘ lobulated cystic lesion, (clear borders and made up of many lobules) and with high density. They are lined with thin, stratified layers squamous epithelium and filled with keratin, which usually appears as a thick, flaky, (scaly) while 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‘, but would look like rust flakes or scales on a mammogram.
The ‘scaling‘ or ‘flaking‘ which 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 related, confusing terms for epidermoid cyst: Pilar cyst and sebaceous cyst
Epidermoid cysts is a term that in the past has frequently been 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, the ‘contents‘ of a sebaceous cyst are different from epidermoid cysts. They are filled with a clear oily liquid made by sebaceous (grease) glands.
They can also contain some amounts of the white, flaky, semi-solid Keratin-like discharge found in epidermoid cysts, and a mixture of oily and white-flaky junk is sometimes called ‘sebum‘. But in modern usage, the term sebaceous cyst is used more specifically for cysts containing primarily the oily discharge of the sebaceous glands.
Breast cysts with infection are harder to differentiate radiologically
Sebaceous cysts of the breast are frequently located in the inflammatory fold, axillary, and medial areas. If the cyst has been complicated by an infection, 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
A ‘Pilar‘ cyst, is another sub-type of epidermoid cyst, often described using the same all-inclusive term. Pilar cysts largely contain keratohyalin and not sebaceous-gland oils, but the cyst ‘lining‘ is different than with epidermoid cysts, as it is made up of ‘hair-root‘ cells, and not skin cells.
For further reading, I suggest you visit this page which has some information on sebaceous cysts of the breast, as well as this page for mucoepidermoid breast carcinoma.
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- Sebaceous cysts: Causes – MayoClinic.com". http://www.mayoclinic.com/health/sebaceous-cysts/
- Zuber TJ (2002). "Minimal excision technique for epidermoid (sebaceous) cysts". Am Fam Physician 65 (7): 140912, 14178, 1420.
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