Skin Biopsy, Excision & Simple Repairs

Choosing and performing a skin biopsy or excision in general practice, and the margins and referral thresholds for keratinocyte cancer. See Cryosurgery Technique for cryotherapy, Local Anaesthesia for infiltration dosing, and Wound Assessment & Laceration Repair for suturing technique.

Choosing a technique

The most common techniques are punch, shave, excisional and incisional biopsy — each yields different information and suits different lesions.

• Punch biopsy is the preferred method for most inflammatory dermatoses (rashes) — it samples full thickness from epidermis to subcutaneous fat. A 4 mm punch is adequate for most histopathology; a smaller punch risks insufficient tissue and should be reserved for cosmetically sensitive sites.
• Shave biopsy suits a raised, solitary lesion where pathology is confined to the epidermis — superficial basal cell carcinoma (BCC), squamous cell carcinoma (SCC) or seborrhoeic keratosis. It is not appropriate for a suspicious pigmented lesion, since partial sampling can miss atypia or malignancy and makes margin assessment harder to interpret.
• Excisional biopsy removes the entire lesion with a margin of normal skin and is the standard for a suspected melanoma or keratoacanthoma, where partial sampling risks missing invasive disease.
• Incisional biopsy samples only part of a lesion and suits deep inflammatory processes (panniculitis), ulcers (biopsied at the junction of ulcer and adjacent normal tissue), cutaneous lymphoma or a lesion too large to excise fully without unacceptable cosmetic cost.

Choose established, representative lesions over early or secondarily changed ones (crusted, scarred, excoriated), and avoid cosmetically sensitive sites, the lower leg (poor healing) and the axilla/groin (higher infection risk) when an alternative site is available.

Biopsy technique

Mark the site with a surgical pen before infiltrating — local anaesthetic can obscure the lesion’s edge. 0.5 mL of anaesthetic is usually enough for a punch or shave biopsy; too much can distort the histology and mimic dermal oedema. Clean the skin gently so as not to disturb overlying scale, which can itself carry diagnostic information; avoid chlorhexidine near the eyes or ears.

Punch: advance to the point of a “giving way” sensation as the punch reaches subcutaneous fat, then free the specimen with iris scissors or a scalpel blade rather than crushing it with forceps. A punch of 4 mm or more usually needs one or more interrupted sutures; 3 mm or smaller can often heal by secondary intention. Never take a partial punch biopsy of a pigmented lesion.

Shave: cut at 45° to the skin at the lesion’s edge, then flatten the blade to the skin surface and continue across until the lesion is removed. Haemostasis is with pressure, aluminium chloride or sulphate solution, or electrocautery — take care combining electrocautery with an aluminium chloride solution in alcohol, which is flammable. Shave wounds heal by secondary intention.

Excisional/incisional: for a pigmented lesion, take the entire lesion with a 2 mm margin of normal skin, down to subcutaneous fat. An incisional biopsy needs only a 1 mm margin of normal skin at one end of the ellipse but should still extend deep into fat.

Send the specimen in the correct medium — 10% formalin for routine histology, saline-soaked gauze or Michel’s medium for direct immunofluorescence, and a sterile container for microbiological culture — and complete the request form with a clinical description and differential, not just “biopsy for histology”; incomplete clinical information is one of the more avoidable causes of a report that doesn’t match the clinical picture.

Excising a keratinocyte cancer

Assess the tumour’s clinical border in good light, ideally under magnification or dermatoscopy — stretching the skin and, for a BCC, gently rubbing it to provoke a flush both help define the true edge. Dot the margin with a surgical marker on adjacent normal skin, then join the dots into a continuous line before adding the excision margin around it. Palpate the surrounding skin to identify the natural direction of closure before cutting.

Margins:

Lesion	Margin
BCC, low-risk	2–3 mm
BCC, high-risk	>5 mm
SCC, low-risk	4 mm
SCC, high-risk	6 mm

High-risk SCC features are size >20 mm, depth >4 mm, recurrence, a head/neck location, perineural or lymphovascular invasion, poor differentiation, or immunosuppression. The excision itself is cut outside the pen markings, with the lesion excised as a circle and the resulting ellipse then completed.

Referral

Refer to a dermatologist or plastic surgeon for lesions with high-risk SCC features (above), a head/neck location, an infiltrating or sclerosing histological subtype, size over 2 cm, or by patient preference. Mohs micrographic surgery, curettage and cautery, cryotherapy, topical therapy (imiquimod) and radiotherapy are all reasonable alternatives to excision depending on lesion and patient factors, and are a specialist discussion rather than a first-line general-practice choice for most of these indications.

Closure

Direct closure is preferred where the defect allows it; flap repair or grafting are options for larger defects and are specialist procedures.