When does a cyst become dangerous?

A cyst is an abnormal lump that develops on a tissue or organ of the body. Cysts appear as closed pockets containing a liquid, a semi-solid substance or, more rarely, air. The most common skin cysts are the sebaceous cysts. They are caused by the rupture of the pilosebaceous follicles, the structures in the dermis where hair originates and where sebum is secreted. This rupture leads to an accumulation of sebum and keratin, a fibrous protein essential to the skin, in the sebaceous canal, which causes its obstruction. Cysts appear as a hard and rounded subcutaneous mass.

Most cysts are benign, meaning they are not cancerous and do not spread to surrounding tissues.

However, certain clinical signs should raise concern and may indicate a potential danger of a cyst:

• The cyst grows rapidly : A cyst that enlarges quickly over a few weeks or months needs to be closely monitored. Indeed, rapid growth or a sudden increase in volume may indicate a proliferation of cancerous cells. Only radiological examinations can differentiate a benign cyst from cancer. If in doubt, a biopsy is necessary to establish a diagnosis.

• The cyst is constantly painful : A cyst that becomes painful may indicate inflammation or a transient infection. These can cause an increase in the internal pressure of the cyst, thereby causing localised pain. The inflammation or infection of a cyst are not dangerous phenomena in themselves. However, it can happen that they do not spontaneously resolve and spread to the surrounding tissues, leading to infectious cellulitis. In this case, the use of antibiotics, sometimes accompanied by the drainage of the cyst, is necessary.

• The cyst changes colour : A change in the hue of the cyst could be a sign of complication. A cyst that turns dark blue or black, for instance, may indicate the presence of blood within the cyst. This phenomenon occurs when the pressure inside the cyst increases to the point of causing the rupture of small blood vessels. It is then recommended to seek medical advice to rule out any risk of complications.

• The cyst becomes hard and appears to adhere to the surrounding tissues : A cyst that becomes hard and seems to adhere to the underlying structures should also raise concern. Unlike benign mobile cysts, a cyst that appears anchored to the surrounding tissues may indicate a malignant tumour invading neighbouring structures. This infiltration occurs when cancer cells spread to adjacent tissues, making the cyst less mobile. The presence of such adhesion may suggest a tumour process. A biopsy once again allows for confirmation.

At the slightest doubt regarding the danger of a cyst, the best reflex to have is to seek advice from a health professional.

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• CAFFREY T. & al. Early Detection and Treatment of Skin Cancer. American Family Physician (2000).

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• DOUGLAS M. & al. Diagnosing Common Benign Skin Tumors. American Family Physician (2015).