Squamous Cell Carcinoma starts as a group of atypical cells in sun-exposed areas, such as head and neck in 70% of the cases (mostly the lower lip, external ear, and periauricular region, but also forehead and scalp), and arms and hands in the remainder. At this early stage, the lesion presents as actinic keratosis (i.e., a flat, scaly, and pink macule). This is precancerous but may not evolve into Squamous Cell Carcinoma. As the atypical cells fill the entire epidermis but do not breach the basal layer, the lesion becomes Squamous Cell Carcinoma in situ (Bowen’s disease). The next and final step is progression to invasive Squamous Cell carcinoma , wherein the atypical cells penetrate below the basement membrane and into the dermis.
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