Actinic keratoses may show extension down into follicles, not only in cases with full-thickness epidermal atypia (bowenoid actinic keratosis), but also in cases with atypia limited to the epidermal basalis. Previous studies have demonstrated that, in bowenoid actinic keratosis, follicular extension is usually superficial, being limited to the upper follicular segment. Little is known about the depth of follicular involvement in cases of invasive squamous cell carcinoma (SCC) of the skin arising from actinic keratosis and the role of the follicle in invasive SCC pathogenesis.
A recent study by Fernández-Figueras et al published in the Journal of the European Academy of Dermatology and Venereology examined the relationship between follicular extension of atypical keratinocytes in an actinic keratosis and the development of invasive SCC from the follicular wall.
The depth of follicular extension was correlated with the depth of invasion of the SCC. Differences between the differentiated and classical pathways of invasive SCC were also examined.
Researchers performed a retrospective histologic review of 193 biopsy specimens of invasive SCC with an associated actinic keratosis, assessing the presence and depth of follicular extension of atypical keratinocytes in the actinic keratosis, using tumor (Breslow) thickness and the follicular unit level (infundibular, isthmic, and subisthmic), as well as invasive SCC being present directly adjacent to the follicular basalis.
Findings of Note
Follicular extension was present in 25.9% of cases (n = 50), usually extending into the lower follicular segment. The invasive SCC was present directly adjacent to the follicular basalis in 58% of cases (n = 29), correlating highly with the depth of follicular extension (infundibular: 3 of 12; isthmic: 21 of 33; subisthmic: 5 of 5).
Researchers point out it is important to note the presence and the depth of follicular extension when diagnosing an actinic keratosis, as follicular extension likely accounts for a significant proportion of recurrent keratoses and the development of invasive SCC following superficial treatment modalities.
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