Latest study made by researchers at Cedars-Sinai Medical Center may explain why cancers are so difficult to treat. According to their research, more than 2,000 genetic variations can appear in just one tumor, so current methods of performing biopsies may not be enough.
http://www.techtimes.com/articles/190672/20161231/over-2-000-genetic-mutations-can-be-present-in-tumors-of-the-same-cancer-type-study.htm
Over 2,000 Genetic Mutations Can Be Present In Tumors Of The Same Cancer Type: Study