Melanocytic naevi

(Cancer Council Australia Melanoma Guidelines Working Party, 2020)

Melanocytic naevi (moles) are benign localised proliferations of melanocytes on the skin or mucosae. Most develop in childhood and adolescence, but they can be present at birth (see Congenital melanocytic naevi, also known as birthmarks). The number of melanocytic naevi is related to sun exposure in childhood, and latitude of residence—on average, a fair-skinned Australian has 50 to 100 melanocytic naevi. Although a single naevus is unlikely to become malignant, having many naevi is a strong risk factor for melanoma.

Most naevi begin as macules (flat pigmented patches). Over time they can become raised, often with gradual loss of pigment. Skin type affects the appearance of melanocytic naevi, and people with darker skin types have more densely pigmented naevi. Naevi can be categorised according to appearance, for example:

  • junctional naevi are flat, and are usually brown-to-black
  • intradermal naevi (the most common type of naevi in adults) are lightly pigmented or skin-coloured domed papules
  • compound naevi combine features of junctional and intradermal naevi, presenting as raised pigmented lesions.

Naevi can develop a rim of depigmentation (ie a halo), caused by the lesion regressing. This is usually a benign phenomenon, but melanomas can also develop depigmented haloes. When assessing a pigmented lesion with a depigmented halo, focus on the lesion and not the halo.

Atypical (or dysplastic) naevi are a benign subset of naevi with atypical appearance. They are rare before adolescence, and can continue to appear throughout life. The appearance of atypical naevi can vary within one patient. Atypical naevi typically:

  • are larger than an average melanocytic naevus
  • have flat and raised components
  • have irregular smudgy borders
  • vary in colour (usually shades of brown and pink)
  • occur on the trunk (but can occur elsewhere).

Naevi rarely need treatment—most melanomas arise de novo, so prophylactic removal of benign naevi is not advised. A benign naevus can be excised for cosmetic reasons or if it is irritating. Commonly, domed dermal naevi are removed by shave excision, but they recur in approximately 20% of cases. Traumatised or recurrent naevi can mimic melanoma, clinically and histologically—inform the pathologist when sending such lesions for histology.

Note: If melanoma is suspected, excise the whole naevus with a 2 mm margin.

Signs suggesting that a naevus needs biopsy to exclude melanoma or rule out melanoma in situ are listed in Melanoma warning signs. If melanoma is suspected, excise the whole naevus with a 2 mm margin, rather than sampling it with a partial biopsy; this means that even if the biopsy shows dysplasia, no further treatment is needed.

Patients with multiple atypical naevi must have regular skin examinations, at a frequency that takes into account their other risk factors for melanoma. See also Skin cancer assessment. Commercially available serial photography and dermoscopy are not substitutes for regular clinical examination by an experienced practitioner.