Wednesday, June 13, 2007

Seeing Tumors with Quantum Dots

Glowing nanoparticles could help doctors make sure they don't leave behind any traces of brain tumors during surgery.

By Katherine Bourzac

Glowing tumors: Fluorescent quantum dots injected into live rats build up in brain tumors, but not in the surrounding tissue. This image combines a white-light photograph of the brains with an infrared image in which blue is the lowest level of intensity and red is the highest. Quantum dots are concentrated in a tumor in the brain on the left; the brain on the right, a control, contains no tumors. Credit: Marcel Bruchez, CMU

Researchers at Carnegie Mellon University (CMU), in Pittsburgh, are using fluorescent nanoparticles to image tumor tissue during biopsies and surgeries. The imaging technique, which is being tested in rodents, could be particularly useful for precisely spotting tumors during surgeries to remove glioblastomas, one of the most common and aggressive forms of brain cancer. On average, patients survive less than a year after their diagnosis with this deadly form of brain cancer, in part because of the difficulty of surgically removing the entire tumor.

Led by CMU chemist Marcel Bruchez and Steven Toms, director of neurosurgery at the Geisinger Clinic, in Danville, PA, the researchers took crisp fluorescent images of brain tumors, called gliomas, in rats. The rats had been injected with nanoparticles that emit infrared light when they are excited by visible light. The infrared rays made by the nanoparticles can be picked up by a small camera and viewed by surgeons. These quantum dots have a core made of cadmium and telluride, surrounded by a zinc-sulfide shell, which is in turn surrounded by a protective polymer coating.

"This particular type of tumor is poorly distinguishable," says Bruchez. And when removing brain tumors, surgeons can't cut wide margins, or patients might lose brain function.

Surgeons removing a brain tumor now rely on a recently taken picture from magnetic resonance imaging (MRI) to orient themselves. But, says Bruchez, "the consistency of the brain is like a bowl of Jell-O. Once you start cutting and removing tissue, things move around, and you can't rely on presurgery imaging." He says that surgeons leave behind glioma tissue more than half the time.

One solution to the problem is to run several MRI scans on the patient during the surgery. With this extra guidance, one study showed, surgeons can remove about 15 percent more glioma tissue. But operating rooms that incorporate MRI are expensive, and surgeons must use special tools that won't be affected by the MRI magnet.

Bruchez and Toms found that glowing quantum dots injected into a rat's bloodstream are brought to glioma tissue--but not to other areas of the brain--by immune cells called macrophages. These cells engulf debris like nanoparticles and swarm to infected and cancerous tissues as part of the body's inflammatory response. The macrophages do not go to healthy brain tissue.