Mechanisms of metastasis

Metastasis is the process by which a primary malignancy establishes distant and discontiguous disease. It is a dreaded and ominous event that usually portends a worse prognosis, with greater tissue destruction, organ dysfunction, and potential mortality. Fortunately, metastasis is a rare event for nonmelanoma skin cancers (NMSC). Basal cell carcinomas (BCC) comprise the vast majority of NMSC (60-80%) and have a metastasis rate of only 0.0028%. For squamous cell carcinomas (SCC), the rate of metastasis is 2-6% and rises with certain high-risk features. Malignant melanoma (MM) is perhaps the most feared among common skin malignancies. It has a marked propensity for metastasis, and deaths from MM far outnumber the combined mortalities of both BCC and SCC. The 5-year survival of localized stage IA melanoma is 95%. This decreases to 67% for nodal disease and less than 20% for distant metastasis. Even more aggressive than MM are rare cutaneous tumors, such as Merkel cell carcinoma and malignant fibrous histiocytoma. This chapter will discuss the mechanisms of metastasis as they apply to cutaneous malignancies, particularly melanoma.