Presentation: skin lumps

Differential diagnosis

Skin and mucous membranes: fibroepithelial polyp (skin tag), epidermoid (sebaceous) cyst, implantation cyst, sebaceous hyperplasia, mucocele, hypertrophic scar and keloid, warts and papillomas, pox virus lumps (molluscum contagiosum, orf, milkers nodules), seborrhoeic keratoses, dermatofibroma, solar/actinic keratosis, keratoacanthoma, and malignant tumours (basal cell carcinoma, squamous cell carcinoma, Bowen disease, malignant melanoma, Kaposi sarcoma, secondary tumour).

Subcutaneous and deeper structures: lipoma, neurofibroma, lymph node, and musculoskeletal (ganglion, bursae).

Three common lumps — distinguishing clinical features

Lipoma. A very common benign, slow-growing tumour of mature adipocytes that grows in the subcutaneous tissue (rarely deep to the fascia or intramuscularly), most often on the trunk and upper limbs. Typically a solitary, painless, soft or rubbery, often lobulated nodule 1cm to over 10cm across; the overlying epidermis is unchanged and not tethered to the lesion. Around 5–10% of patients have multiple lesions, usually linked to an underlying syndrome (familial multiple lipomatosis, Proteus syndrome, Gardner syndrome, PTEN hamartoma syndrome, MEN2B); angiolipomas are frequently painful, unlike a typical lipoma. Diagnosis is usually clinical — imaging or biopsy is reserved for diagnostic uncertainty. Differentials include epidermoid cyst, hibernoma, liposarcoma, angiolipoma, angiomyolipoma and ganglion cyst. Surgical excision is curative provided the thin capsule is completely removed; liposuction or laser lipolysis gives a better cosmetic result but a significantly higher recurrence rate since the capsule is not fully removed.

Epidermoid (sebaceous) cyst. A benign cyst arising from the infundibulum of a hair follicle, encapsulated in a thin epidermis-like lining and filled with keratin and lipid-rich debris — “sebaceous cyst” is a common misnomer, since it does not involve sebaceous glands or contain sebum. Presents as a firm, flesh-coloured or yellowish papule or nodule (1–3cm) fixed to the skin surface but typically mobile over deeper layers, often with a central punctum from which foul-smelling cheesy debris can be expressed; most common on the central trunk and face, and twice as common in men. Rupture — from trauma or infection with Staphylococcus aureus, E. coli or group A streptococcus — causes swelling, redness and tenderness. Diagnosis is clinical; biopsy is not usually required, but histological examination of any excised specimen is recommended because of the small risk that a cutaneous squamous cell carcinoma has arisen within the cyst. Key differentials: a trichilemmal cyst (firm, mobile, 0.5–5cm, no central punctum, usually on the scalp), acne pseudocyst, myxoid pseudocyst (digit), and dermoid cyst (epidermal and dermal components, arising in early childhood). Complete surgical excision with an intact capsule is the most effective treatment; incomplete removal risks recurrence.

Dermatofibroma. A common benign fibrous nodule (also called a fibrous histiocytoma), most often found on the lower legs and more common in women. Presents as a solitary firm papule or nodule, usually 7–10mm (range 0.5–1.5cm), tethered to the skin surface but mobile over subcutaneous tissue; pinching the lesion produces the characteristic dimple (pinch) sign, and colour ranges from pink/light brown in white skin to dark brown or black in dark skin. Usually asymptomatic, though sometimes tender or itchy. Diagnosis is usually straightforward clinically, supported by dermoscopy (classically a central white area with a faint surrounding pigment network). Diagnostic excision or biopsy is warranted for an atypical feature — recent enlargement, ulceration, or asymmetrical structures/colours on dermoscopy — since these can mimic dermatofibrosarcoma protuberans or desmoplastic melanoma. Reassurance is usually all that is needed; surgical removal carries a real recurrence risk since the lesion often extends beyond its clinical margin.

A diagnostic approach

Follow the routine of look, feel, move, measure, auscultate, transilluminate. Describe: number, site, shape (regular/irregular), size (metric units), position, consistency (very soft to stony hard), solid or cystic, mobility, surface/contour, and special features (superficial/deep attachment, exact anatomical site, relation to nearby structures and overlying skin, colour, temperature, tenderness, pulsation, reducibility, percussion, fluctuation, bruit, transillumination).

Establish which tissue layer the lump sits in: skin (moves when the skin is moved, e.g. epidermoid cyst); subcutaneous (skin moves over the lump); muscle (mobile when relaxed, fixed on contraction); tendon/joint (movement of these structures changes the lump’s mobility/shape); bone (immobile, best outlined with muscles relaxed).

Features prompting biopsy or referral

No single feature is diagnostic on its own, but each of the common benign lumps above has a recognised malignant mimic, and the features that should prompt biopsy or referral are consistent across them: rapid growth or recent enlargement, ulceration, fixation to underlying structures, an unusually hard or stony-hard consistency, and — where dermoscopy is used — an asymmetric structure or colour rather than the lesion’s classic pattern. A dermatofibroma with any of these features can mimic dermatofibrosarcoma protuberans or desmoplastic melanoma; a lipoma that is growing, painful or deep/large enough to raise doubt should prompt consideration of liposarcoma; and an epidermoid cyst carries a small but real risk that a squamous cell carcinoma has arisen within it, which is part of why histology is recommended on any excised specimen rather than assuming a punctum and cheesy discharge settles the diagnosis outright.