Melanoma and Non-Melanoma Skin Cancer
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Direct answer
Melanoma is a malignant tumour of melanocytes whose prognosis depends most on Breslow thickness, the depth in millimetres from the granular layer to the deepest tumour cell. Basal cell carcinoma is the commonest skin cancer and rarely metastasises, whereas squamous cell carcinoma has greater metastatic potential. The ABCDE criteria help clinicians recognise melanoma early.
What you must remember
- ABCDE criteria: asymmetry, border irregularity, colour variegation, diameter above 6 mm, and evolution over time.
- Breslow thickness: the strongest prognostic factor in localised melanoma, measured with a micrometer from the granular layer.
- Ulceration: an adverse feature that raises the staging category, even in thin melanomas.
- Sentinel lymph node biopsy: considered from 0.8 mm thickness (T1b) and usually recommended above 1 mm, or for thinner melanomas that are ulcerated.
- Basal cell carcinoma: the commonest skin cancer, with pearly rolled edges and telangiectasia; locally invasive and rarely metastasises.
- Squamous cell carcinoma: keratin pearls and an ulcerating nodule; it can metastasise and arises from actinic keratosis, chronic wounds or immunosuppression.
- Melanoma risk factors: intense UV exposure, fair skin, dysplastic naevi, family history and xeroderma pigmentosum.
- Acral lentiginous melanoma: the commonest subtype in darker skin, arising on palms, soles and nail beds.
How the three cancers differ in origin and behaviour
The three commonest cutaneous malignancies differ in cell of origin, appearance and spread, and examiners test these differences directly. The table compares the features that most often appear in questions.
| Feature | Melanoma | Basal cell carcinoma | Squamous cell carcinoma |
|---|---|---|---|
| Cell of origin | Melanocyte | Basal keratinocyte | Epidermal keratinocyte |
| Typical look | Pigmented macule or nodule with irregular border | Pearly papule with rolled edge and telangiectasia | Scaly, keratotic or ulcerating nodule |
| Metastasis | Common, can occur early | Rare | Possible, especially from lip and ear |
| Key prognostic marker | Breslow thickness and ulceration | Site, size and histological subtype | Depth of invasion and site |
| Usual treatment | Wide local excision by thickness | Excision or Mohs micrographic surgery | Excision with margins by risk |
Melanoma is the malignancy whose outcome depends on staging, so depth and ulceration decide the investigations that follow. Basal cell carcinoma is locally destructive but rarely spreads, whereas squamous cell carcinoma can metastasise, especially when it arises in chronic wounds or on the lip.
Diagnostic traps in skin lesions
Most errors come from misreading the ABCDE criteria or from using the wrong biopsy technique.
- Trap: using the 6 mm diameter as a cutoff that excludes melanoma Correct: diameter is one criterion among several, and small melanomas occur, so evolution and the other features matter.
- Trap: treating basal cell carcinoma as a metastasising tumour Correct: it is locally destructive and rarely spreads, whereas squamous cell carcinoma is the non-melanoma cancer that metastasises.
- Trap: using a shave biopsy on a suspicious pigmented lesion Correct: an excisional biopsy with a narrow margin is preferred so that Breslow thickness can be measured accurately.
- Trap: requesting sentinel node biopsy for every melanoma Correct: it is reserved for lesions that meet the thickness or ulceration criteria, because it is an invasive staging procedure.
Frequently asked questions
What is the most important prognostic factor in localised melanoma?
Breslow thickness is the most important prognostic factor in localised melanoma, measured in millimetres from the granular layer to the deepest tumour cell. Ulceration upstages the tumour, and mitotic rate adds prognostic information. Thickness determines the staging category and the need for further investigations.
Which is the commonest skin cancer?
Basal cell carcinoma is the commonest skin cancer, most often seen on the face of older adults. It is locally invasive and rarely metastasises. Wide excision or Mohs micrographic surgery is chosen according to the site and size of the lesion.
What are the criteria for sentinel lymph node biopsy in melanoma?
Sentinel lymph node biopsy is considered for melanomas of 0.8 mm or more and is usually recommended above 1 mm, and it is also considered for thinner melanomas that are ulcerated. It stages the regional lymph nodes and informs further treatment and follow-up. It is not indicated for every melanoma, so the thickness and ulceration criteria are applied first.
Which skin cancer shows keratin pearls on histology?
Squamous cell carcinoma shows keratin pearls on histology, along with irregular nests of atypical squamous cells. It typically presents as a scaly or ulcerating nodule, often on sun-exposed skin or in a chronic wound. Unlike basal cell carcinoma, it can metastasise and is staged accordingly.