Actinic keratoses (AKs) typically arise on chronically sun-damaged skin and represent the most common lesions in the spectrum of keratinocyte skin cancer. Clinically they present as multiple pink macules or papules with a variably scaly surface.
Nonpigmented actinic keratosis
Nonpigmented AK on the face show four dermoscopic features, allowing an accurate diagnosis with high sensitivity and specificity:
- Erythema: Structureless pale-red Areas without any recognizable areas of hypopigmentation
- Pink-to-Red pseudonetwork: structureless red areas that resemble Network structure- small white areas correspond to follicular openings of the skin
- Fine, wavy vessels (straight or coiled) surrounding the hair follicles
- Targetoid hair follicles: Hair follicle openings surrounded by a white halo and filled with a yellowish keratotic plug (white circle surrounding a yellow clod)
On nonfacial skin, AK usually exhibits nonspecific patterns, with features such as surface scale and keratin and dotted vessels (tiny red dots densely aligned next to each other).
An additional clue to the diagnosis of AK is the rosette sign, which can only be seen with polarized light and consists of a white four-leaf clover-shaped structure. Although the rosette sign can be seen in actinic damaged skin and in tumors such as basal cell carcinoma (BCC) and melanoma, they are more commonly encountered in AK and thin SCCs.
Pigmented actinic keratosis
Pigmented variants of AK (pAK) mostly occur in darker skin phototypes. The discrimination between benign pAK and pigmented tumors such as Lentigo maligna (LM) may be challenging, due to the dermoscopic overlapping morphological features. The clinical differentiation from LM may be possible through palpation; pAK are most commonly having a rough texture with scaly-appearing surface and sharp border.
The reported dermoscopic characteristics of the pAK include:
- Evident follicles: Hair follicle openings of different sizes.
- White circles: White ring-like structures within the hair follicle. The follicle may have a targetoid appearance with central yellowish keratotic plug surrounded by a white halo
- Grey rhomboidal structures: Grey confluent dots arranged in lines or grey-to-brown linear structures located between follicular structures
The dermoscopic differential diagnosis of pAK is supported by the presence of a prominent pseudonetwork located between keratin-filled ostial openings. The dermoscopy of LM may on the other side reveal asymmetrical pigmented follicular openings and a darker dot located within ostial openings (some call this the isobar sign); a finding rarely seen in pAK.
The dermoscopy may help guide the best location to biopsy. Biopsying areas which reveal the most suspicious features, such as annular–granular structures, asymmetric follicular openings, dots within the ostial openings, or rhomboidal structures may provide an accuse histologic diagnosis.
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