Daniel, 71 years old. Erythematosquamous lesions.

The patient has actinic keratoses (AKs) on the scalp. These lesions are common in older people with a light phototype. They are most often visible via their superficial component and are scaly or keratotic. They cause cosmetic concerns and are usually asymptomatic.

When they first appear, they are erythematous lesions with fine scales. They are pre-carcinomatous lesions which are considered to be skin markers of chronic UV exposure. A patient with AK therefore has an increased risk of developing skin carcinoma or melanoma.

The scalp of men suffering from alopecia is an area at risk of AK. AKs include a keratosis of varying thickness that reforms when detached, on an inflammatory base.

In the case of multiple AKs, we speak of field cancerisation, particularly on the scalp of bald men, where there is also marked helioderma. The diagnosis is primarily clinical.

In the case of carcinomas, the lesions are much more infiltrated, tumour-like or even ulcerated. However, the clinical distinction between AK and squamous cell carcinoma in situ (Bowen's disease) or micro-invasive carcinoma is sometimes difficult to make, justifying a biopsy in case of doubt.

In psoriasis, there are erythematosquamous plaques with thick whitish scales. The onset age is typically younger, and there is often a personal history of psoriasis and/or other lesions indicative of psoriasis (plaques on the elbows, knees, lower back…).

The lesions in contact dermatitis are acute, very pruritic, ill-defined, oozing and often appear following the application of a topical product on the affected area.

In the case of seborrheic dermatitis, the onset age is much younger, the lesions are found in the hairy areas, and there is diffuse dandruff associated with an oily scalp.

It is indeed the cumulative exposure to the sun throughout life that gives rise to actinic keratoses, which typically appears after decades of sun exposure in elderly individuals.

UV exposure is considered the primary risk factor for AK. Prolonged sun exposure associated with outdoor activities, or the use of sunbeds increase the risk of developing AK.

Immunosuppression is a risk factor for the development of AK, which can be observed in people who have received an organ transplant or suffer from certain hematologic disorders.

The most frequent locations are:
- The head (particularly on the nasal bridge, but also on the cheekbones, forehead and temporal regions).
- Neck.
- Forearms.

Most often they are areas with marked helioderma. The areas with helioderma on which AKs develop constitute field cancerisation, which means that there are visible cancerous and precancerous lesions, but also infra-clinical lesions, characterised by histological or purely molecular abnormalities at the periphery of visible lesions.

The main risk of AKs is that they can develop into invasive lesions of squamous cell carcinoma. A high number of AKs is associated with an increased risk of developing squamous cell carcinoma (SCC).

Actinic keratoses can regress spontaneously.

Liquid nitrogen cryotherapy is a fast method and the most frequently used, involving the application of a soaked cotton swab, in one or more sessions, or spraying with a cryospray.

Preventive removal of AKs is recommended because of the risk of degeneration into squamous cell carcinoma. Several treatment options are available. AKs can be removed with instruments or pharmaceutical products.

PDT involves the application of a topical photosensitising agent which, when illuminated by blue or red light, leads to the selective destruction of the AKs.

5-FU is an anti-metabolite cytotoxic agent from the antipyrimidine family. It is applied once or twice a day, depending on tolerance, for 3 to 4 weeks, to the lesion(s), extending at least as far as the surrounding area.

Erbium:Yttrium aluminium garnet (Er:YAG) and CO2 ablative lasers are indicated for larger isolated lesions or small numbers of AKs.

Any keratosis resistant to treatment or with suspicious features (induration, pain, size greater than 1 cm, rapid growth, bleeding or ulceration) should be biopsied to rule out squamous cell carcinoma.

The histological appearance includes dysplasia of the basal layers of the epidermis with irregular, hyperchromatic nuclei, parakeratosis, and almost always lymphocytic dermal inflammation. There are hypertrophic, atrophic, pigmented, and lichenoid variants. The main anatomical-clinical problem is invasion of the dermis, which defines primarily micro-invasive squamous cell carcinoma.

Along with other non-invasive imaging techniques, dermoscopy can help in the decision to biopsy.