Three major types of skin cancer originate from cells in the epidermis. In general, skin cancer is caused by unpro-tected, long-term exposure to the sun’s ultraviolet radiation. The most common type is the basal cell carcinoma, which microscopically, as its name implies, resembles cells from the stratum basale of the epidermis. Basal cell carcinoma is a slow-growing tumor that usually does not metastasize. Typically, the cancer cells arise from the follicular bulge of the outer root sheath of the hair follicle. Almost in all cases of basal cell carcinoma, the recommended treatment is surgical removal of the tumor. The second most common skin cancer is the squa-mous cell carcinoma with more than 200,000 cases each year. Individuals with this form of cancer usually develop a small painless nodule or patch that is surrounded by an area of inflammation. Squamous cell carcinoma is charac-terized by highly atypical cells at all levels of the epidermis (carcinoma in situ). Disruption of the basement membrane results in spread (metastasis) of tumor cells to the lymph nodes. Squamous cell carcinoma is known for variable dif-ferentiation patterns ranging from polygonal squamous cells arranged in orderly lobules and zones of keratinization to rounded cells with foci of necrosis and occasional single keratinized cells. Treatment for squamous cell carcinoma depends on histological type, size, and location. It may in-clude surgical excision, curettage and electrodesiccation, cryotherapy (freezing with liquid nitrogen), or chemoor ra-diotherapy. Moh’s micrographic surgical procedure is being used for locally recurrent skin cancers. This proce-dure involves shaving away one-by-one thin layers of epi-dermis and examining them under a microscope for the presence of malignant cells. When the shaving is cancer free, the surgery is complete. This method preserves as many unaffected skin layers as possible while making cer-tain that all cancer cells are removed. Malignant melanoma is the most serious form of skin cancer if not recognized at an early stage and surgically removed. Individual melanoma cells, which originate from melanocytes, contain large nuclei with irregular contours and prominent eosinophilic nucleoli. These cells either aggregate in nests or are scattered through the entire thick-ness of the epidermis (Fig F15.1.1). They may reside only in the epidermis (melanoma in situ) or extend into the underly-ing papillary layer of the dermis. With time, the melanoma undergoes a radial growth phase. The melanocytes grow in all directions, upward in the epidermis, downward into the dermis, and peripherally in the epidermis. At this early stage, the melanoma tends not to metastasize. On the skin surface, it presents itself as an irregularly pigmented multicolor lesion, appearing black with parts brown to light brown, and a mixture of pink to red or shades of blue (Fig. F15.1.2). In time (approximately 1 to 2 years), melanocytes exhibit mitotic activity and form round nodules growing perpendicularly to the surface of the skin. In this vertical growth phase, the melanocytes display little or no pigment and usually metastasize into regional lymph nodes.
 FOLDER 15.1 Clinical Correlation: Cancers of Epidermal Origin