Melanoma presents
a case of extremes.

If melanoma is detected while it is still local (called melanoma in situ), the five-year survival rate is over 95%. And, fortunately, about 80% of melanomas are diagnosed in this early stage.
If the cancer is more advanced, however, survival drops below 60%. Its deadly nature is due to the spread of cancerous melanoma cells to other parts of the body. Cancer cells that spread in this fashion are known as metastases and are very difficult to treat. Melanoma cells usually spread first via lymph vessels (channels that carry immune system cells and fluids to the lymph nodes) or glands. Melanoma cells can also spread via blood vessels to various organs, causing metastatic tumors in the liver, lungs, brain, or other sites.
In general, after patients are treated for melanoma, the longer they remain free of cancer recurrence following treatment, the better the chance of remaining disease-free. However, relapses are not uncommon in those whose initial melanoma was large. (Current research suggests that even many local melanomas may be more dangerous than previously thought. Even after small melanomas have been completely removed, patients must be monitored for recurrence.) .