Finding a new lump beneath the skin often raises concerns about cancer. Growths commonly referred to as sebaceous cysts are overwhelmingly benign, meaning they are non-cancerous. A cyst is a closed sac or capsule under the skin filled with fluid or semi-solid material, and these are among the most common dermatological findings. The vast majority of these slow-growing skin nodules are harmless and typically do not require treatment unless they become inflamed or cause discomfort.
Defining the Lumps: Sebaceous Versus Epidermoid Cysts
The term “sebaceous cyst” is widely used by the public, but it is often a misnomer for what is actually a much more frequent type of growth called an epidermoid cyst. Epidermoid cysts originate from the hair follicle, forming when surface skin cells become trapped beneath the skin’s surface and multiply instead of shedding.
These trapped cells then produce keratin, a thick, cheese-like protein substance that fills the cyst sac. This accumulation of keratin gives the contents of an epidermoid cyst their characteristic color and texture. Epidermoid cysts are common on the face, neck, and trunk, and frequently have a visible dark opening or punctum on the skin’s surface.
A true sebaceous cyst, on the other hand, is significantly rarer and originates from the sebaceous glands, which are responsible for producing sebum, the skin’s natural oil. These cysts form when the duct of a sebaceous gland becomes blocked, causing sebum to accumulate in a sac. The contents are therefore more liquid or oily, unlike the keratin found in an epidermoid cyst.
Because of this distinction in origin and content, dermatologists now prefer the term epidermoid cyst for the common keratin-filled lumps. Despite the technical difference, both types of cysts are generally slow-growing, movable beneath the skin, and classified as benign skin adnexal tumors.
The Direct Answer: Malignancy Risk in Benign Cysts
The chance of a common benign skin cyst, whether epidermoid or true sebaceous, developing into a cancerous lesion is exceptionally low. This transformation is considered an extremely rare event. The overwhelming majority of cysts removed and examined are confirmed to be completely benign, posing no risk of local spread or metastasis.
When malignant transformation does occur, it is usually confined to the wall of a long-standing cyst. The types of skin cancer most frequently reported to arise in association with these cysts are basal cell carcinoma and squamous cell carcinoma. Squamous cell carcinoma, which originates from the trapped epidermal cells, is the more common of the two rare occurrences.
The incidence rate of malignancy arising within an epidermoid cyst is negligible in the general population. Studies of cysts that were already excised due to atypical features suggest a transformation rate of less than 1%, reinforcing the benign nature of these growths. The primary issues associated with these growths remain inflammation, infection, or cosmetic concern, rather than malignancy.
When to Seek Professional Evaluation
While the risk of cancer is very low, it is important to seek professional evaluation for any new or changing lump to confirm the diagnosis. A medical professional can distinguish a common cyst from other types of benign growths, such as a lipoma, or, more rarely, a true tumor. The typical diagnostic process involves a simple visual inspection and physical examination of the lump’s size, texture, and mobility.
There are specific signs that a previously stable cyst may be experiencing complications and should be seen by a doctor promptly. These include signs of infection or acute inflammation, such as a sudden increase in redness, warmth, or tenderness to the touch. The presence of pus or foul-smelling discharge from the cyst also indicates a likely bacterial infection that requires medical attention.
A rapid increase in the size of the lump or any significant change in its shape or texture warrants a visit to a dermatologist. A non-healing sore or an area of thickened, irregular skin developing on the cyst’s surface can make a doctor consider the possibility of a more complex or malignant condition. In cases where the lump is highly suspicious or the diagnosis is unclear, a biopsy may be performed to definitively rule out malignancy.