Clinical meaning
Skin cancers arise from different cell types in the epidermis and have vastly different behaviors and prognoses. Basal cell carcinoma (BCC) is the most common cancer overall, arising from basal cells in the stratum basale. It grows very slowly, almost NEVER metastasizes (<0.1%), and typically presents as a pearly, flesh-colored papule with telangiectasia (visible blood vessels) and rolled borders, often on sun-exposed areas (face, ears, nose). It may develop central ulceration ('rodent ulcer'). Squamous cell carcinoma (SCC) arises from keratinocytes in the stratum spinosum. It is the second most common skin cancer and CAN metastasize (3-5% risk, higher in immunosuppressed patients). SCC presents as a scaly, erythematous plaque or nodule, often with a rough, crusted surface, and may develop a cutaneous horn. Risk factors include cumulative UV exposure, immunosuppression, chronic wounds/scars (Marjolin ulcer), and HPV infection. Actinic keratoses are pre-cancerous SCC precursor lesions. Melanoma arises from melanocytes and is the most dangerous skin cancer due to its high metastatic potential. It accounts for only 4% of skin cancers but causes 75% of skin cancer deaths. The ABCDE criteria for melanoma detection: Asymmetry (one half unlike the other), Border irregularity (scalloped, notched), Color variation (multiple shades of brown, black, red, white, blue), Diameter >6 mm (larger than a pencil eraser), Evolving (changing in size, shape, color, or symptoms). Breslow depth (measured in millimeters from the top of the granular layer to the deepest point of tumor invasion) is the single most important prognostic factor for melanoma — deeper invasion correlates with higher metastatic risk and worse survival.