3.2.1 Seborrhoeic Keratosis

Seborrhoeic wart, senile wart, basal cell papilloma.

The seborrheic keratosis is a benign verrucous epithelial tumour, without any viral association, typically seen in elderly patients but that may also occur in young people. It is the most frequent benign tumour of the skin.

The cause of seborrheic keratosis is not known, but somatic mutations in FGFR3 (and other genes such as PIK3CA) are frequently found. Stable and clonal mutations or activation of FRFR3, PIK3CA, RAS, AKT1 and EGFR genes are frequently found in seborrhoeic keratoses.

Various clinical phenotypes exist: generally, it is a soft, wart-like (verrucous) keratotic brown or black plaque of the skin with a “pasted-on” appearance. However, it can have a lighter, yellowish colour or be flat. Some lesions may be pedunculated. The warty surface may be covered with a thick, squamous, keratotic film. The lesions can vary in size from a few millimetres to several centimetres. If exposed to physical contact, it can get irritated or inflamed and be pruritic.

Several histological variants exist. Major criteria are acanthosis, hyperkeratosis and horn pseudo- cysts, usually accompanied by epidermal hyperpigmentation. There are different histological types of seborrheic keratosis, such as the acantholytic, hyperkeratotic, reticulated or adenoidal forms. The presence of moderate atypia or mitosis is suggestive of irritation or local inflammation. In such cases, a dermal, perivascular, diffuse or lichenoid lymphocytic infiltrate may be present.

If seborrheic keratoses appear on areas prone to physical injury, they may become irritated, infected or may be bleeding. Itching is often related to M. furfur colonization. Malignant transformation is extremely rare. Leser-Trélat sign is the explosive onset of multiple, itchy seborrheic keratosis, and may indicate an underlying solid tumour, most often stomach cancer or a colon adenocarcinoma.

Seborrheic keratoses can be identified by clinical features. Dermoscopy may be helpful in visualising the horn pearls in particular when naevi are a differential diagnosis. Histology is generally not necessary except for melanoacanthoma type.

Treatment of seborrheic keratosis is generally not needed except in traumatized and infected ones. Recurrence after treatment is very frequent. Treatment options include curettage, cryosurgery, laser ablation, shave excision.