Actinic Keratosis

Solar keratoses are also known as actinic keratoses. The adjective solar is more specific because it refers to the sun as the cause, whereas the adjective actinic refers to a variety of rays . Even among the sun's rays, action spectrum evaluations indicate that the ultraviolet B (UVB) rays (290 to 320 nm) are the most damaging ("carcinogenic") rays, although UVA rays (320 to 400 nm) can augment the damaging effects of UVB rays .

Actinic keratoses are usually seen as multiple lesions in sun-exposed areas of the skin in persons in or past middle life who have fair complexions .Excessive exposure to sunlight over many years and inadequate protection against it are the essential predisposing factors. Actinic keratoses are seen most commonly on the face and the dorsa of the hands and in the bald portions of the scalp in men .

Usually, the lesions measure less than 1 cm in diameter. They are erythematous, are often covered by adherent scales, and, except in their hypertrophic form, show little or no infiltration. Some actinic keratoses are pigmented and show peripheral spreading, making clinical differentiation from lentigo maligna difficult . Occasionally, lesions show marked hyperkeratosis and then have the clinical aspect of cutaneous horns. A lesion analogous to actinic keratosis occurs on the vermilion border of the lower lip as solar cheilitis and may show areas of erosion and hyperkeratosis.

Actinic keratosis and solar cheilitis can develop into squamous cell carcinoma. However, the incidence of this transformation is difficult to determine because the borderline between actinic keratosis and squamous cell carcinoma is not clear

patients with actinic keratoses squamous cell carcinoma develops in one or more of the lesions . Usually, squamous cell carcinomas arising either in actinic keratoses or de novo in sun-damaged skin do not metastasize. The incidence of metastasis in different series varies from 0.5% to 3%             

In carcinoma of the vermilion border of the lip, however, metastases have been found in 11 % of the cases .

Histopathology

Actinic keratoses are keratinocytic dysplasias or squamous cell carcinomas in situ. This definition is preferable to their designation as precancerous because most of them never progress to cancers. Biologically, the lesions are still benign; invasion into the dermis, if present at all, is limited to the most superficial portion, the papillary dermis (see Differential Diagnosis) .

This variety of keratosis, sometimes referred to as florid keratosis, may easily be overdiagnosed as invasive squamous cell carcinoma by the unwary. Mild or moderate papillomatosis may be present. The epidermis is thickened in most areas and shows irregular downward proliferation that is limited to the uppermost dermis and does not represent frank invasion .A varying proportion of the keratinocytes in the stratum malpighii show a loss of polarity and thus a disorderly arrangement. Some of these cells show pleomorphism and atypicality ("anaplasia") of their nuclei, which appear large, irregular, and hyperchromatic. Often the nuclei in the basal layer are closely crowded together. Some of the cells in the midportion of the epidermis show premature keratinization, resulting in dyskeratotic cells or apoptotic bodies characterized by homogeneous, eosinophilic cytoplasm with or without a nucleus. In contrast to the epidermal keratinocytes, the cells of the hair follicles and eccrine

ducts that penetrate the epidermis within actinic keratoses retain their normal appearance and keratinize normally. Occasionally, cells of the normal adnexal epithelium extend over the atypical cells of the epidermis in an umbrella-like fashion. In some cases, abnormal keratinocytes extend downward on the outside of the follicular infundibulum to the level of the sebaceous duct and, less commonly, along the eccrine duct .

A variant of the hypertrophic type of actinic keratosis is the lichenoid actinic keratosis, which demonstrates nuclear atypia, irregular acanthosis and hyperkeratosis, the presence of basal cell liquefaction, degeneration of the basal cell layer, and a bandlike "lichenoid" infiltrate in close apposition to the epidermis . Fairly numerous eosinophilic, homogeneous apoptotic bodies are seen in the upper dermis as so-called Civatte bodies. Aside from the presence of nuclear atypicality,     

there is considerable resemblance to lichen planus and benign lichenoid keratosis. An additional distinguishing feature may be the presence of plasma cells in the lichenoid infiltrate.

In rare instances of actinic keratosis of the hypertrophic type, in addition to finding anaplastic nuclei in the lower epidermis, one finds areas of epidermolytic hyperkeratosis in the upper epidermis. These changes are like those seen in bullous congenital ichthyosiform erythroderma, in linear epidermal nevus, and as incidental epidermolytic hyperkeratosis in a variety of lesions. In areas of epidermolytic hyperkeratosis, one observes in the upper epidermis clear spaces around the nuclei and a thickened granular layer with large, irregularly shaped keratohyaline granules (39). Epidermolytic hyperkeratosis may occur also in

lesions of solar cheilitis

.