Even a small amount of tumor tissue can be deadly. For children with medulloblastoma, for example, if the surgeon leaves behind less than 1.5 cubic centimeters of cancer tissue, the survival rate is 80 percent. If the surgeon leaves more than this amount, survival drops as low as 50 percent. But distinguishing small amounts of tumor tissue during surgery is difficult, especially during brain surgery.
Olson says that near-infrared imaging could be used at what is normally the endpoint of cancer surgeries, when doctors, guided by preoperative images and their own sense of touch and sight, believe they have removed the extent of a tumor. Surgeons could then wheel an infrared imaging system up to a patient who has been administered the fluorescent tumor label, then find and remove any remaining traces of cancer.
Using their near-infrared imaging agent in mice, the University of Washington researchers were able to detect clusters of as few as 200 cancer cells. Olson says that in his 15 years of oncology he has seen “nothing else close” to this sensitivity. This resolution is “pretty impressive,” agrees Khalid Shah, head of the molecular neuroscience imaging lab at Massachusetts General Hospital’s Center for Molecular Imaging Research.
The high resolution of the tumor paint should help surgeons see which tissues to remove–and also what to leave behind. This is a particular problem during brain surgery, when removing normal tissue can mean loss or impairment of major functions. But removing cancerous tissue while sparing normal tissue is a difficult trade-off during all cancer surgeries, says Olson. Surgeons must “decide whether or not to do extensive surgery and take out lots of lymph nodes,” which carry cancer cells from their original site to other places throughout the body. This is a difficult choice: while removing lymph nodes can prevent cancer from coming back, loss of many lymph nodes can cause swelling, aching, and numbness long after patients recover from surgery.
The advantage of the University of Washington imaging agent is its specificity, says Michael Egan, president of TransMolecular, the company that makes chlorotoxin. TransMolecular will work with Olson to move the agent to clinical trials. Olson and Egan are confident that the agent will prove safe for use in humans, given the success of chlorotoxin in human trials and their results in the mouse study. Autopsies and evaluations of the mice’s organ function showed no evidence of toxicity.