Suspected melanoma: how to biopsy
The case
A 45-year-old woman with Fitzpatrick skin type II and a history of several severe childhood sunburns presents with an 8-month history of a pigmented lesion on her upper back. It has grown, developed irregular borders and now varies in colour from brown to black with areas of blue. She measures it at about 8 mm. There is no itch, bleeding or ulceration. Examination confirms asymmetry, border irregularity, colour variation and elevation. What is the most appropriate next step?
Options
- AUrgent excisional biopsy with 2 mm margins
- BSuperficial shave biopsy for histology
- CDermoscopy and observe whether atypia resolves
- DPunch biopsy of the most irregular area
- ETopical imiquimod for 6 weeks
Think it through before you read on. Which single option is best, and why are the other four wrong?
Show the answer and explanation
Answer
A. Urgent excisional biopsy with 2 mm margins
Why this is the right answer
Australia has the highest melanoma rate in the world, and this lesion has all the warning features: Asymmetry, Border irregularity, Colour variation (including blue), Diameter over 6 mm and Evolution over months. In a fair-skinned woman with a history of sunburn, this is melanoma until proven otherwise.
The Cancer Council Australia melanoma guidelines recommend that a suspicious pigmented lesion is removed whole by excisional biopsy with a 2 mm clinical margin and a cuff of subcutaneous fat [VERIFY]. This removes the entire lesion for histology and allows the pathologist to measure Breslow thickness, which is the single most important prognostic factor and determines the margins of the later wide local excision.
Shave biopsy cuts across the lesion and can transect a melanoma, making Breslow thickness impossible to measure. Punch biopsy samples only part of the lesion; the deepest part may be missed, and staging will be wrong.
The 2 mm margin is deliberately narrow. Wider margins at this stage would disturb lymphatic drainage and compromise a sentinel node biopsy if one is needed later.
If melanoma is confirmed, definitive wide excision follows: margins of 5 mm for in situ disease, 1 cm for tumours up to 1 mm thick, and 1 to 2 cm for thicker lesions, with sentinel node biopsy considered for tumours over 1 mm (or over 0.8 mm with ulceration) [VERIFY].
Why the other options are wrong
- B
Shave biopsy can transect the tumour and destroys the ability to measure Breslow thickness.
- C
Suspicious dermoscopic features do not resolve; observation delays diagnosis of a potentially invasive cancer.
- D
A partial sample may miss the deepest part and understage the melanoma.
- E
Imiquimod is for lentigo maligna or superficial BCC after histological diagnosis, not for an undiagnosed suspicious lesion.
High-yield takeaway
Any lesion suspicious for melanoma is excised whole with a 2 mm margin; never shave, punch or observe it.
Reference: Cancer Council Australia Clinical Practice Guidelines for the Diagnosis and Management of Melanoma (2023)
Common questions
Why a 2 mm margin and not wider?
Wider margins disturb lymphatic drainage and can compromise a later sentinel node biopsy. The definitive wide excision comes after histology.
What determines the final excision margin?
Breslow thickness: 5 mm for in situ disease, 1 cm for tumours up to 1 mm, and 1 to 2 cm for thicker melanomas.
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