The most dangerous aspect of melanoma is its ability, in later stages, to spread (or metastasize) to other parts of the body. The term 'metastatic melanoma', also known as stage IV melanoma, is used when melanoma cells of any kind (cutaneous, mucosal or ocular) have spread through the lymph nodes to distant sites in the body and/or to the body's organs. The liver, lungs, bones and brain are most often affected by these metastases.
As with other cancers, metastases occur when the melanoma is not caught in the early stages. Oftentimes, symptoms only become present once it is has already spread. Once melanoma has spread, determining the original type is nearly impossible – which makes planning the right treatment extremely difficult. Early detection is crucial.