Benign Melanocytic Lesions

Features of benign pigmented lesions

• Well defined margin

• Even pigmentation

• Symmetrical

• Not changing over time or changing very slowly

Freckles (ephelis)

• Patchy increase in melanin pigmentation which occurs after UV exposure

• Most common in fair skinned and red heads

• Reflects clumpy distribution of melanocytes

• People with freckles have one defective copy of MC1R gene

Actinic lentigenes

• Also known as 'age' or 'liver' spots

• Related to long term UV exposure

• Tend to be present on the face, forearms and dorsal hands

Histology

• Elongated rete ridges in the epidermis

• Increase in melanin and basal melanocytes

Melanocytic naevi (common moles)

Congenital melanocytic naevi

• Visible pigmented (melanocytic) proliferations in the skin that are present at birth

• Affects of 1-2% babies born

• Junctional - naevus cells cluster in the dermoepidermal junction

• Tends to be flat or slightly elevated with smooth surface, uniform pigmentation

• Categorised as small (<2cm diameter), medium (2-20cm diameter), and (giant > 20cm diameter)

• Larger lesions have a 10-15% risk of melanoma with increase in age - may need to be removed by staged surgical excision

• Larger lesions often show hypertrichosis

Usual type acquired naevi

• During infancy the melanocyte: keratinocyte ratio breaks down at a number of cutaneous sites, which allows the formation of simple naevi

• Compound - naevus cells at epidermal junction and upper dermis

• Lesions slightly elevated or dome shaped, often pigmented

• Hairs may project from surface

• Exact process of naevi induction unknown but may be immune regulated - immunosuppressed leukaemic children have more naevi

• Average person has 20-30 naevi, mostly aquired 1st-2nd decades

Intradermal naevi

• Naevus cells in dermis - all junctional activity ceased, entirely dermal

• Dome shaped, verrucous (warty), pedunculated, or sessile, often flesh coloured, occasionally hairy and may display surface telangiectasia

Dysplastic naevi (DN)

• Generally >6mm diameter, with varied pigment and border asymmetry

• Most common

• Not inherited

• One or several benign acquired melanocytic neoplasms

• Risk of malignant melanoma slightly raised

• Autosomal inheritance of a high penetrance gene e.g. CDKN2A means a patient will develop lots of atypical naevi - need to be removed

• Lifetime melanoma risk can be up to 100%

• Biopsy will show architectural atypia and cellular atypica, with a host reaction of fibrosis and inflammation

• Epidermis not effaced (unlike melanoma)

• Severe dysplasia may be difficult to distinguish from melanoma in situ

Rarer naevi

• Halo naevi: have a peripheral halo of depigmentation
• Show inflammatory regression and are overrun by lymphocytes

• Blue naevi: entirely dermal and consist of pigment rich dendritic spindle cells

• Spitz naevus: usually occur <20 years, consist of large spindle and/or epithelial cells, pink colour due to prominant vasculature, may closely mimic melanoma (epidermal hyperplasia) but most Spitzoid lesions are benign

Halo naevus

Spitz naevus

 

Blue naevus