Seeing Tumors with Quantum Dots

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Bruchez wants to build an imaging system that's compatible with standard operating procedures. Outfitting an operating room for infrared imaging of tumors would involve adding an infrared digital camera and installing filters on the lights to eliminate ambient infrared light, ensuring that the only infrared light in the room comes from quantum dots. The quantum dots can be tuned to emit visible light, which would eliminate the need for the imaging system, but doctors would have to turn off the lights to see the glow from tumors, then turn them back on and readjust their eyes to continue with surgery.

Bruchez says that he and his collaborators have had good results using a system based on the technology to perform surgical removal of tumors in rodents, although these results are as yet unpublished. He expects the work to apply generally to various types of cancers; other unpublished research by his group, he says, demonstrates that the tendency of macrophages to gobble up quantum dots and travel with them to tumor tissue holds for many other cancers.

Bruchez and Toms are also developing biopsy needles with optical imaging systems. Brain-tumor biopsies are normally time consuming and hit or miss. "You go where you believe the tumor to be, take out a sample, send it to the pathology lab, and wait in the operating room for the results," says Bruchez. If the surgeons missed the tumor, they have to take another sample and wait for the results again. If a patient were first injected with tumor-seeking quantum dots that could be detected by the biopsy needle, the process might be that much easier.

The safety of quantum dots for brain imaging still needs to be investigated. The cores of the quantum dots used by Bruchez are made of cadmium, and although there is no evidence so far that cadmium-containing quantum dots are toxic, some researchers are wary. "Cadmium is very toxic," points out Wenbin Lin, a chemist at the University of North Carolina who is developing nanoparticles for high-contrast MRI. "We need to worry about that."

Indeed, says Bruchez, "there are concerns with cadmium, but the formulation that is used for fluorescence imaging should render the cadmium nonbioavailable." The organic polymers surrounding the cadmium can't be broken down by typical biological processes, he says. However, to address toxicity concerns, Bruchez says he is developing imaging techniques that require lower doses of the particles and encapsulation methods that render the metal even less bioavailable.