ABSTRACT
Despite recent efforts at high throughput molecular characterization of tumors,
histopathology remains the gold standard for diagnosis of cancer in general,
and of cancer in the skin in particular. Skin cancers, like other cancers, are classi-
fied largely on the basis of their cell or tissue of origin (1). Cancers of the skin can
arise from any of the many cell types that are present within skin, and are thus
very diverse. The most important skin cancer, in terms of mortality, is melanoma.
However, squamous cell and basal cell carcinomas (often lumped together as
“nonmelanoma skin cancer”) are much more common than melanomas in Cau-
casian populations. Squamous cell carcinoma, which may arise from surface epi-
dermis, usually with an associated precursor lesion called an actinic keratosis, is
perhaps the prototypic skin cancer (2). Actinic keratoses are potential precursors
of squamous cell carcinoma and are also markers of individuals at increased risk
of skin cancer (including melanoma) (3). Basal cell carcinoma, more common
than squamous cell carcinoma, usually does not arise in association with an
evident precursor. These lesions can be divided into subtypes that correlate
with risk of local recurrence and, rarely, metastasis (4). Basal cell and squamous
cell carcinoma are exceedingly common in populations with susceptible fair skin
who live in sunny climates or work or vacation in the sun. These nonmelanoma
skin cancers are often not collected by tumor registries, and are often treated
locally, yet because of their large numbers, they contribute substantially to mor-
bidity and also to mortality from skin cancer.
