Recent research by Cancer Research UK has found that breast cancer can actually be classified as ten different diseases. The study looked at 2,000 frozen samples of breast tumour tissue taken over ten years and put into ten different subtypes, and therefore treatment can be better targeted to the particular subtype, and a more specific diagnosis means that treatment could be more effective.
For more information from an NHS article, see:
http://www.nhs.uk/news/2012/04april/Pages/breast-cancer-genetic-diversity-mapped.aspx