For many cancer patients, survival depends on whether their doctors can successfully remove the tumors in their entirety. But relying on conventional imaging technologies makes this difficult, and surgeons often inadvertently leave cancer cells behind.

Researchers at the University of Washington's Fred Hutchinson Cancer Research Center have developed an imaging agent that allows physicians to identify and surgically remove smaller areas of tumor tissue than is possible with standard imaging techniques. In tests, the fluorescent imaging agent helped them detect several kinds of cancer in mice, including those of the brain, prostate, and intestines. The researchers hope to begin testing the imaging system in humans in as few as 18 months.

The researchers call the agent "tumor paint" because, when injected into the bloodstream, it seems to cling to and illuminate only tumor cells. The tumor paint allows for what's called molecular imaging: it tells researchers about a tissue's chemical activities, rather than showing them what a tissue looks like, as do traditional imaging technologies such as magnetic resonance imaging (MRI). Molecular imaging for cancer could help doctors find and remove traces of tumor cells that are invisible during surgeries guided by preoperative MRIs, the current standard.

The tumor-paint compound has two major parts. One is a protein called chlorotoxin, which binds to cancer cells. This protein, which is made by TransMolecular, a company based in Cambridge, MA, is currently in phase II clinical trials as a targeted treatment for brain cancer. To make chlorotoxin visible to near-infrared cameras, the University of Washington researchers bound it to a fluorescent protein. Near-infrared light can travel only one or two centimeters through the body, which means the University of Washington agent couldn't be used to diagnose cancer noninvasively in people. But doctors could use near-infrared imaging during surgery to remove a tumor, or use an endoscope to look for colon cancers.

Researchers developing molecular imaging agents for cancer have run up against two major challenges, says James Olson, who led the imaging research and is a practicing physician and professor of pediatric oncology at the University of Washington. It has been difficult to synthesize imaging agents that bind only to cancer cells. And most that have been developed are limited to detecting only one cancer type.

The University of Washington agent solves both problems. It seems to target an enzyme that breaks down the tissues that surround and hold tumors in, enabling them to grow and spread. This enzyme is made by many tumor types.

Olson says that performing near-infrared imaging during surgery could have a great impact on the treatment of tumors that are not sensitive to radiation and chemotherapy, including many prostate, ovarian, and breast cancers. For many common cancers, what determines how long a patient survives is whether doctors succeed in completely removing a tumor. "You assume clusters [of cancer cells] are left behind for all [these surgeries]," says Olson.