http://www.medscape.com/viewarticle/570624 explains it well
The abstract--
Nat Med. 2008 Feb 24 [Epub ahead of print]
Noninvasive assessment of cancer response to therapy.
Han Z, Fu A, Wang H, Diaz R, Geng L, Onishko H, Hallahan DE.
Department of Radiation Oncology, School of Medicine, Vanderbilt University, 1301 22nd Avenue South, B902 TVC, Nashville, Tennessee 37232, USA.
Rapid assessment of cancer response to a therapeutic regimen can determine efficacy early in the course of treatment. Although biopsies of cancer can be used to rapidly assess pharmacodynamic response, certain disease sites are less accessible to repeated biopsies. Here, we simultaneously assess response in all sites of disease within days of starting therapy by use of peptide ligands selected for their ability to discern responding from nonresponding cancers. When conjugated to near-infrared imaging agents, the HVGGSSV peptide differentiates between these two types of cancer. Rapid, noninvasive assessment of the pharmacodynamic response within cancer promises to accelerate drug development and minimize the duration of treatment with ineffective regimens in cancer patients.
PMID: 18297085 [PubMed - as supplied by publisher]