Environmental influences 

A mutation can also be caused by external factors. 
Continuous exposure to some substances found in food, tobacco or the environment (such as hormones or pesticides) increases the chance of mutations.

Some types of radiation can also trigger changes in the cells.
An example is solar ultraviolet radiation, which have been linked to melanoma and skin cancer.

However, very rarely can a single mutation lead to cancer

Typically, small mutations accumulate over a long time, changing the gene structure little by little until the cell displays a dangerous behavior.

This is one of the reasons why most cancers tend to appear late in life, ordinarily after the age of 50.