When removing malignant tumors, it is important for surgeons to know that they have removed all the cancerous cells. This is especially important in the brain, where more than 80 percent of malignant cancers recur around the edges of the surgical site. However, this task is hard because surgeons rely on the appearance and feel of tissue during surgery, and today’s imaging tools, such as MRI, can distinguish tumors from healthy tissue only when more than 1 million cancer cells are present.Researchers at Fred Hutchinson Cancer Research Center and at the University of Washington, both in Seattle, have developed a fluorescence-based technique that might one day help surgeons more precisely see whether they have removed all the cancerous cells. For this, they created a conjugate of the chlorotoxin peptide and the IR-emitting fluorophore CY5.5. The conjugate, which they call tumor paint, can identify tumors with as few as 2000 cancer cells.The team, led by Dr. James M. Olson, demonstrated that, in mouse models, the technique could reveal brain tumors as small as 1 mm without causing surrounding normal brain tissue to fluoresce. The conjugate is advantageous because it activates within hours, it begins binding to cancer cells within minutes, and the signal from the cancer cells lasts 14 days. The work was published in the July 15, 2007 issue of Cancer Research. The researchers are preparing toxicity studies before seeking approval from the FDA to begin clinical trials. If successful, the chlorotoxin:Cy5.5 conjugate could help surgeons not only remove all cancerous cells, but also avoid healthy tissue. The investigators think that the conjugate also might find use as a noninvasive screening tool for early detection of skin, cervical, esophageal, colon and lung cancers.