Squamous cell carcinoma (SCC) has peak incidence at age 60. Risk factors includechronic sun exposure (ultraviolet UVB); fair complexion; chronic skin ulcers or sinus tracts; long-term exposure to hydrocarbons, arsenic, burns, and radiation; immunosuppression; and xeroderma pigmentosum. Common mutations include TP53 and HRAS.
· Precursors include actinic keratosis (a sun-induced dysplasia of the keratino-cytes that causes rough, red papules on the face, arms, and hands) and Bowen disease (squamous cell carcinoma in situ).
· Squamous cell carcinoma occurs on sun-exposed areas (face and hands) and causes a tan nodular mass which commonly ulcerates. Microscopic examina-tion shows nests of atypical keratinocytes that invade the dermis, (oftentimes) formation of keratin pearls, and intercellular bridges (desmosomes) between tumor cells. Squamous cell carcinoma of the skin rarely metastasizes and complete excision is usually curative.
· A variant is keratoacanthoma (well differentiated Squamous cell carcinoma), which causes rapidly growing, dome-shaped nodules with a central keratin-filled crater; these are often self-limited and may regress spontaneously.
Basal cell carcinoma (BCC) is the most common tumor in adults in the Westernworld; it is most common in middle-aged or elderly individuals and arises from the basal cells of hair follicles. Risk factors include chronic sun exposure, fair complexion, immunosuppression, and xeroderma pigmentosum.
BCC occurs on sun-exposed, hair-bearing areas (face), and may form pearly pap-ules; nodules with heaped-up, translucent borders, telangiectasia, or ulcers (rodent ulcer). Microscopically, BCCs show invasive nests of basaloid cells with a palisading growth pattern.
BCC grows slowly and rarely metastasizes, but it may be locally aggressive. Shave biopsies have a 50% recurrence rate, but complete excision is usually curative. Muta-tions affecting the Hedgehog pathway are seen in sporadic and familial cases.