3.3.6 Lentigo Maligna

Hutchinson melanotic freckle; lentigo maligna melanoma in situ; premalignant melanosis of Dubreuilh; Hutchinson freckle.

Lentigo maligna represents 10-15% of all melanoma with a slight female predominance. Incidence is 3.84/100,000/y. Mean age around 70 years.

Lentigo maligna is an intraepithelial proliferation of atypical melanocytes occurring mostly in sun-damaged skin and in elderly patients. LM is defined as melanoma in situ (MIS) on chronically sun-damaged skin. If the lesion becomes invasive, it is termed lentigo maligna melanoma (LMM).

Lentigo maligna occurs after natural and artificial chronic UV exposure, and it accounts for about 5-15% of melanomas and 10-26% of head and neck melanomas, accounting for a larger percentage of melanomas occurring in patients over the age of 65 years. The progression rate of LM to LMM is not known but it is estimated to be about 5%. The 5 years survival rate about 97-100%. LM/LMM is more likely to harbor mutations in KIT, compared to other subtypes of melanoma, in which BRAF mutations are more common. Other mutations are in CCND1, MITF, NRAS, and p53.

They are most frequently located on sun-exposed areas of the face and neck and scalp in bald patients but they can also appear in the trunk and extremities of patients with chronic sun damage.

Precursor of lentigo maligna melanoma.

Histology shows epidermal atrophy, increased number of atypical melanocytes in basal layer (also extending into hair follicle), dermal lymphocytic infiltrate, melanophages, and solar elastosis. Specific inmunostainnings of melanocytes are recommended for the diagnosis of LM and evaluation of margins after surgery.

Lesions are slowly extending and finally develop into a lentigo maligna melanoma. Lesions located at the mucosal borders at the eye lids and lips may show a higher risk of progression.

After several years, lesions may become infiltrated and nodular, and may evolve towards an invasive malignant melanoma with metastasis.

Diagnosis relies on clinical features, histology and dermatoscopy.

Lentigo maligna must be differentiated from pigmented actinic keratosis, lentigo simplex, solar lentigo, flat seborrhoeic keratoses, melanoma (lentigo maligna melanoma, superficial spreading melanoma), naevoid lentigo, or atypical naevi.

The treatment depends on the size and location. Surgical excision is preferred. Mohs micrographic surgery with special stainning of the atypical melanocytes reduces the risk of recurrence and spares benign skin tissue compared to conventional surgery with wider margins. If excision is not possible or otherwise contraindicated, alternatives are soft X-ray treatment, or topical imiquimod (off label).