Lung cancer can lie dormant for more than 20 years before turning deadly, helping explain why a disease that kills more than 1.5 million a year worldwide is so persistent and difficult to treat, scientists said.

Two papers detailing the evolution of lung cancer reveal how after an initial disease-causing genetic fault — often due to smoking — tumor cells develop numerous new mutations, making parts of the same tumor genetically unique.

By the time patients are sick enough to be diagnosed with cancer, their tumors will have developed down multiple evolutionary pathways, making it very hard for any one targeted medicine to have an effect.