Scientists at Scripps Research Center have invented a test that they believe can detect cancer cells in the blood that have broken away from the primary tumor , CTC circulating tumor cells,
 
Doctors hope to use CTC s for early detection of cancer and to quickly monitor whether  treatments are working effectively 
 
Ideally doctors would like to attain cancer cells from a patients blood,  do whole genome sequencing on them to determine treatment options,  expand the cancer cells in a dish ,  try different cancer drugs on them to see which will work best before using them on the patient and then monitor the treatments success with additional CTC blood sample biopsies. 
 
http://www.marketwatch.com/story/team-led-by-scripps-research-scientists-demonstrates-effective-new-biopsy-in-a-blood-test-to-detect-cancer-2012-02-03
 
http://articles.nydailynews.com/2011-12-20/news/30540190_1_cancer-cells-cancer-treatment-options-technique
 
http://medicalxpress.com/news/2012-03-single-cell-sequencing-era-cancer.html
http://www.medicalnewstoday.com/releases/240253.php