Abstract: Basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC), as certain forms of non-melanoma skin cancer (NMSC) or keratinocyte carcinoma, are the most common forms of malignant neoplasms worldwide (Sharp et al., 2024). BCC and cSCC have been identified as two major components of NMSC, comprising one-third of all malignancies (Burton et al., 2016). Generally speaking, patients with NMSC tend to have relatively favorable survival outcomes, while different histopathological subtypes of NMSC exhibit distinct biological behaviors (St?tescu et al., 2023). Keratinocyte carcinoma, although not considered as deadly as melanoma, tends to metastasize if left untreated (Civantos et al., 2023; Nanz et al., 2024). cSCC can evolve locally, then aggressively metastasize, invade, and even lead to fatal consequences in a subset of patients (Winge et al., 2023). A solid, pigmented, smooth plaque or a hyperkeratotic papule with or without central ulceration and hemorrhage appears to be characteristic of cSCC (Thompson et al., 2016; Zhou et al., 2023). Of note, a rare type of intraepidermal cSCC in situ often appears as a velvety, demarcated, slightly raised erythematous plaque on the genitalia of men (Yamaguchi et al., 2016). Accounting for approximately 16.0% of scalp tumors and with a rising incidence, cSCC is now the second most common NMSC in humans (Verdaguer-Faja et al., 2024). According to the latest statistics, up to 2%–5% of cSCCs in situ may gradually progress into invasive cSCCs in the final step (Rentroia-Pacheco et al., 2023). Several risk factors for the carcinogenesis and development of cSCC have been identified, including age, accumulative exposure to ultraviolet light radiation A and B, human papillomavirus infection, arsenic ingestion, chronic scarring, xeroderma pigmentosa, a relevant history of ionizing radiation, androgenetic alopecia in males, and immunosuppression therapy (Martinez and Otley, 2001; Welsch et al., 2012; Mortaja and Demehri, 2023).

Key words: Nonmelanoma skin cancer; Cutaneous squamous cell carcinoma (cSCC); 5-Aminolevulinic acid photodynamic therapy (5-ALA PDT)

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