A pilar cyst is a build up of fluid and keratin just under the skin, that can develop around the root-sheath of hair follicles. A pilar cyst is a sub-type of epidermoid cyst, and they are often described using the same general term. They are also sometimes referred to as a "wen", a "Isthmus-catagen" cyst, or a trichilemmal cyst. Pilar cysts tend to form in areas of high hair concentrations, and about 90% of them form on the scalp. The other 10% tend to be formed in various areas of the body, and rarely on the breast. Essentially a pilar or trichelimma cyst forms when large amounts of keratin are deposited in the 'root sheath' area of a hair follicle, within a short time span.
Pilar cysts are smooth and moveable, and usually a women concerned about a possible breast cancer 'lump' will bring it to the attention of her family physician. Generally speaking, pilar cysts have no symptoms. But, they can become infected, in which case they can become red, swollen, and painful. Pilar cysts are non-cancerous (benign). There are exceedingly rare reports of pilar cysts which become cancerous, but if a woman brings a lump to the attention of breast cancer screening physicians and finds out it is a pilar cyst, this is good news. It is not breast cancer.
The development of a pilar cyst is far more common in women than in men, and they tend to occur in older women the most. About 70% of the time they occur in multiples, and are 'solitary' about 30% of the time. Up to 10% of the population will likely develop one or more pilar cysts at some point in their lives. No one knows exactly what causes a pilar cyst, but it has been suggested that they form due to the 'budding' from the external root sheath (the trichelemma). Pilar cysts are believed to be 'genetically pre-determined', as they tend to run in families.
Very rarely, pilar cysts may show rapid (proliferative) cell growth and become more solid. These are called "pilar tumors'. They are non-cancerous , but they can grow aggressively and become so large that they have to be surgically removed. Usually, the clinical presentation of a proliferative pilar tumor (PPT) is of a long-standing pilar or other epidermoid cyst that slowly progresses to a mass, often with a history of inflammation. Pilar tumors (also called trichilemmomas) are clinically benign, but they can show mitotic cells, nuclear atypia, and dyskeratotic cells (cells which have changed into a 'white-plaque' form), and this can be a misleading (incorrect) indicator of possible skin carcinoma. (However, it would be very unlikely to be mistakenly identified as breast cancer)
On mammogram, a pilar tumor may appear as a lobulated, well-circumscribed mass, and externally the skin may be ulcerated and atrophied. Pilar tumors, occuring on the breast or anywhere else, have to be followed a little more closely. While they are mostly benign, they have been known to grow in a highly proliferative way locally, and even to metastize ( the ability to metastize more or less defines 'malignancy') So, the concern would be that while a proliferative pilar tumor is not really causing any problems, it might start to develop on a critical organ or the brain, where it could cause serious problems. Trichilemmomas usually range in size from 1 to 10 cm in diameter, but, they can really grow sometimes, even reaching sizes up to 25 cm in diameter.
Since pilar cysts are benign, not cancerous, and almost always have no association with breast carcinoma at all, it is really not necessary to treat them. But, they can grow and at a certain point cause a cosmetic concern, so they are sometimes drained, or surgically removed. Of course, if the cyst has become infected, those symptoms will most likely be treated with oral antibiotics.