The key to a successful cancer surgery is to extract every last bit of the tumor. If any cancerous cells are left behind, they could cause the disease to reappear in the same place or close by later on. Imagine how useful it would be if the malignant tissue glowed bright green, practically shouting, “Cut me out, I’m dangerous!”

Turns out, it can.

Penn scientists demonstrated that by using an injectable dye that preferentially accumulates in cancerous tissues, they could make lung tumors glow green under an infrared light. Performing operations on canine and human patients, surgeons were able to distinguish cancer from non-cancer—an ability that could reduce the likelihood of local recurrences. Their findings were published in the journal PLOS ONE.

“Surgeons have had two things that tell where a cancer is during surgery: their eyes and their hands,” says David Holt, the first author on the study and a professor of surgery in the School of Veterinary Medicine. “This technique is offering surgeons another tool, to light tumors up during surgery.”

Holt teamed with researchers from the Perelman School of Medicine, led by Sunil Singhal, an assistant professor of surgery, who tested the technique in human patients.

To differentiate healthy from diseased tissue, the researchers turned to a dye called indocyanine green, or ICG. Approved for use in people by the Food and Drug Administration in 1958, ICG fluoresces bright green under near-infrared light (NIR). It also happens to concentrate in tumor tissues because these cancerous masses have “leaky” blood vessels due to fast rates of growth. The leaky walls let in the dye whereas blood vessels in normal tissue do not.

“Our group has been experimenting with new strategies to use ICG to solve a classic problem in surgical oncology: preventing local recurrences,” Singhal says. “Our work uses an old dye in a new way.”

The team first had to perform proof-of-concept studies in mice with lung cancer to see if the dye worked as intended. After giving the mice ICG and viewing their tumors under NIR, the researchers found they could distinguish tumors from normal lung tissue as early as 15 days after the mice acquired cancer. These tumors were visible to the human eye by 24 days.

The next step was to try the technique in dogs that had developed cancer naturally. The Penn team enrolled eight dogs in the study, representing a diversity of breeds and sizes, from a 6-pound miniature pinscher to a 60-pound Labrador retriever. At Penn Vet’s Ryan Hospital, the dogs received ICG intravenously the day before surgery. During each operation, the surgeon looked at the patient’s chest under NIR both before and after removing the tumor to see if they could view a glow from the tumor tissue and differentiate it from normal tissue. They also examined the tumor itself under NIR after it was extracted.

Holt says the cancerous tissue stood out, glowing green whereas the normal tissue did not.

“Because it worked in a spontaneous large animal model, we were able to get approval to start trying it in people,” he says.

With approval from Penn’s Institutional Review Board, the clinicians moved on to a pilot study in humans at the Hospital of the University of Pennsylvania. Five patients, each of whom had cancer in their lungs or chest, received an injection of ICG prior to surgery. As in the canine surgeries, the surgeon performing the operation on humans examined the patient’s chest during the procedure under NIR, reimaging the tumor after it was removed from the patient.

The surgeons observed that all of the tumors fluoresced strongly, giving them confidence that the technique worked in humans.

Not only was this pilot study a confirmation that the technique could help, it may well have saved the life of one of the five patients enrolled. In this patient, CT and PET scans had indicated that the tumor was a solitary mass. Yet during the procedure, NIR imaging revealed glowing areas in the supposedly healthy parts of the lung.

“It turns out he had diffuse microscopic cancer in multiple areas of the lung,” Holt says. “We might have otherwise called this Stage I, local disease, and the cancer would have progressed. But because of the imaging and subsequent biospy, he underwent chemotherapy and survived.”

Holt, Singhal, and colleagues plan to continue to refine their technique, looking for different NIR dyes that are even more specific to tumor tissue to give surgeons the upper hand in tackling cancer.