This is a FANTASTIC summary of the major discoveries and breakthroughs in cancer over the past 250 years – http://www.cancer.gov/aboutnci/overview/250-years-advances?utm_content=sf25688262&utm_medium=spredfast&utm_source=twitter&utm_campaign=National+Cancer+Institute&cid=sf25688262
Of course, the the targeted therapy era of oncology started in 2001 with Gleevec, which targeted the fusion protein created by the Philadelphia Chromosome translocation. The NCI history of cancer summarizes is thusly – The drug imatinib mesylate, the first anticancer drug developed specifically to target the molecular defect that causes a particular type of cancer, is shown to be effective against chronic myelogenous leukemia (CML). Later, it would also be shown to be effective in the treatment of gastrointestinal stromal tumors (GIST).
I recommend this paper for a thorough discussion of the discovery and its implications for the identification of other targeted oncology therapeutics – http://www.nature.com/scitable/topicpage/gleevec-the-breakthrough-in-cancer-treatment-565