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Actinic keratosis

Actinic Keratosis Types

How is actinic keratosis classified?

Actinic keratosis

Actinic keratosis (AK) is a common precancerous skin lesion that develops on areas that have been chronically exposed to the sun. In dermatology, classification has two main goals. First, it helps describe the clinical and microscopic patterns seen in actinic keratosis. Second, it supports decisions about risk and management.

Classification of AK usually draws on visible features such as texture, thickness, and colour, as well as histopathological details. 

There are two main systems followed to classify actinic keratosis. One widely adopted clinical grading method sorts lesions into three levels: the lowest grade includes lesions that are easier to feel than to see, the next grade covers lesions that are both visible and palpable, and the highest grade is reserved for clearly thickened hyperkeratotic lesions.

The second grading system focuses on how deeply atypical keratinocytes extend within the epidermis. Early-grade lesions show atypia limited to the basal and suprabasal layers. Mid-grade lesions involve roughly half of the epidermis. The highest grade is used when atypia affects more than two thirds of the epidermis.

Although these grading systems are useful, many clinicians also classify AK by histopathological variant subtype, which often provides a clearer picture of behaviour and risk. 

What are the different types of actinic keratosis?

Actinic keratosis (AK) presents in several histological forms. While they vary in appearance and severity, all are considered potentially precancerous and require appropriate attention. Here is a summary of the main histological subtypes of actinic keratosis: 

Hypertrophic

Hypertrophic AK is marked by thickened, scaly skin and prominent surface build-up. Under the microscope, the outer layer often shows alternating patterns of retained nuclei and fully keratinized cells. These lesions are usually easy to feel and see, particularly on areas exposed to strong sunlight, such as the backs of the hands. This subtype is one of the most common and is notable for a higher likelihood of progressing to cutaneous squamous cell carcinoma, largely due to its thickness and degree of cellular abnormality. 

Atrophic

These lesions are thinner and may be flat or only slightly raised. They often blend into sun-damaged skin, which makes them easy to overlook. On examination, they feel dry or gritty, and histology reveals a thin epidermis with early atypical changes at the base of the cells. Although less conspicuous than hypertrophic lesions, atrophic AKs are also common. 

Bowenoid

Bowenoid AK shows atypical keratinocytes throughout the full thickness of the epidermis but without invasion into deeper layers. This pattern resembles squamous cell carcinoma in situ. Clinically, these lesions tend to appear as red, scaly patches or plaques. Because of the depth of abnormality within the epidermis, this subtype requires careful evaluation and management. 

Acantholytic

In acantholytic actinic keratosis, the normal bonds between skin cells are disrupted. This results in small spaces within the epidermis and the presence of abnormal, prematurely keratinized cells. Clinically, it can resemble other skin conditions that involve loss of cell cohesion, so a biopsy is often needed to confirm the diagnosis of actinic keratosis. 

Pigmented

Actinic keratosis that falls into this category contains increased melanin within the epidermis, giving it a brown, tan, or grey-brown colour. Although less frequent, this form is clinically important because it can resemble harmless pigmented lesions or, in some cases, melanoma. Accurate identification is therefore essential.

Among these variants, hypertrophic and atrophic AKs are the most common. Hypertrophic lesions, in particular, need close monitoring because they carry a greater risk of developing into skin cancer. Even so, every actinic keratosis type should be considered potentially precancerous, regardless of thickness or prominence. 

Additional considerations: grades and risk stratification

AK-4

Beyond subtype classification, grading systems help determine risk and guide treatment of actinic keratosis. The commonly used Olsen clinical grading system categorizes actinic keratosis from grade I to III based on visibility and thickness, which aids decisions on treatment intensity.

Histological grading methods, such as that proposed by Röwert-Huber and colleagues, assess how deeply atypical cells extend within the epidermis and can provide an indication of theoretical malignant potential.

ak

When considering the likelihood of malignant transformation, there are several important points to consider:

  • Although the annual risk of a single AK progressing to invasive squamous cell carcinoma is low, the overall risk rises significantly in individuals with many lesions or widespread sun-damaged skin.
  • In higher-risk areas such as the face and ears, some studies suggest an estimated progression risk of just over half a per cent at one year and around two to three per cent at four years.
  • Because it is not possible to predict which lesions will progress, most clinical guidelines advise treating all identifiable AKs and monitoring the surrounding field of sun-damaged skin. 

Recognizing the different types and grades of actinic keratosis is essential for assessing risk and choosing appropriate management. Even though the probability of any single lesion becoming invasive is relatively low, the presence of multiple AKs or extensive sun damage substantially increases overall risk. For this reason, thorough skin examination, timely treatment of visible lesions, and ongoing surveillance of sun-exposed areas form the cornerstone of safe and effective care. 

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