Epidermoid cyst or lipoma
two different lesions

An epidermoid cyst (often called an atheroma) and a lipoma both feel like a “lump under the skin”, but they are different lesions. How to tell them apart, and why both are removed whole.

“A lump under the skin” is the most common description patients give when they come for an examination. Behind it, however, are two different lesions — an epidermoid cyst (often called an atheroma) and a lipoma. Both are benign, but they have a different origin, behave differently and have different features at operation.

What an epidermoid cyst is

An epidermoid cyst forms from the skin. It is a thin-walled sac filled with keratin — a whitish-yellow, greasy material with a characteristically unpleasant smell. It is most often found on the face, behind the ear, on the neck, scalp and back.

• it lies shallowly, directly under the skin and is attached to it;
• it often has a small dark point in the centre — the blocked opening of a hair follicle;
• it grows slowly, but can become inflamed: it turns red, becomes tender and sometimes discharges pus and empties, but then fills up again.

What a lipoma is

A lipoma is a benign lesion made of fatty tissue. It feels like a soft, doughy lump that slides slightly under the fingers. It is most often on the back, shoulders, arms and thighs. It is usually painless and does not become inflamed, but it can grow over the years and become a nuisance with clothing, when lying down or under pressure.

How to tell them apart

• Epidermoid cyst: shallow, firmer, often with a point on the surface, and can become inflamed.
• Lipoma: deeper, soft, mobile, with no point, and rarely becomes inflamed.
• The final diagnosis is made at an examination — sometimes with an ultrasound scan. Not every lump is an epidermoid cyst or a lipoma, so a diagnosis should never be made “by eye”.

What they have in common: both are removed whole

The basic rule for both lesions is the same — they are removed in full, together with their wall or capsule.

• Epidermoid cyst: if even a small part of the wall is left behind, the sac re-forms and the lesion comes back. That is why “squeezing” or only draining it is not recommended — it gives temporary relief but does not cure it.
• Lipoma: it is removed whole, together with its capsule. Remnants of fatty tissue can lead to regrowth. Sometimes there may be no clearly defined capsule, which makes surgical treatment more difficult but does not make it impossible.
• The operation is done under local anaesthesia, through an incision planned to suit the size of the lesion and the skin creases. Often a small piece of skin has to be excised to compensate for the stretched skin once the lesion has been removed. A stitch is placed, and the removed tissue may be sent for histological examination — as with any lesion removed.

When is the best time

For an epidermoid cyst, it is best to operate in a “quiet” period, when it is not inflamed. An inflamed cyst is dealt with first (by treatment or drainage, as judged appropriate), and the full removal is done once the inflammation has settled. If the lesion has been inflamed many times, scarring beneath it makes the operation a little harder, so it is better not to wait.

For a lipoma, the decision to operate is made after an examination, taking into account its size, its location and whether it is growing or causing symptoms. Once that decision has been made, the lipoma is removed whole.

What cannot be promised

• No scar at all — the scar is linear and fine, but its length depends on the size of the lesion.
• An absolute guarantee against recurrence if the lesion has been inflamed many times and its wall has fused with the surrounding tissue.

When to seek a doctor sooner

• a lesion that is growing fast, is hard, fixed or has an irregular surface;
• redness, warmth, pain or pus;
• a lump that is changing or bleeding.