Cancer surgeons strive to remove cancerous cells while preserving as much healthy tissue as possible. Unfortunately, cancer cells are notoriously difficult to identify visually.

A group led by the National Cancer Institute’s Hisataka Kobayashi has developed a fluorescent spray that can label cancer cells within a minute. The hope is that surgeons could apply it during or after a procedure to catch any cancer cells they might have missed.

Several research teams have been working on fluorescent labels for cancer cells that could serve as a visual guide for surgeons, but other methods typically take much longer to work.

The researchers demonstrated the spray’s ability to label cancer cells in mice in a study published last week in Science Translational Medicine. The fluorescence is activated by an enzyme called y-glutamyl transpeptidase that is abundant in tumor cells but not in normal cells. The probe that Kobayashi and his team designed contains a chemical target of the tumor enzyme. The enzyme cleaves the chemical on contact, and this activates the fluorescence signal.

Because the enzyme sits on the cell surface, this reaction occurs within seconds. Fluorescence probes that target molecules inside cancer cells can take several hours or sometimes days to build up enough of a signal. “A probe that is fast like this could really benefit surgeons in the operating room,” says Michael Bouvet, a cancer surgery expert at the University of California, San Diego. Bouvet coauthored an editorial accompanying the study. “Even when you think you’ve taken out all of the primary tumor, you’d be surprised by how much cancer is often left behind.”

Bouvet says the spray approach could be particularly beneficial for ovarian and colon cancers, which can spread into the surrounding cavities, making surgical removal more challenging. While it is possible to label cancer cells with fluorescence in advance, being able to apply the dye locally requires a much lower dose than injection or oral administration—about 1,000 to 10,000 times lower, according to Kobayashi—which would help allay toxicity concerns.

Not all cancer cells express the enzyme that the spray targets, and, of the ones that do, only those that are actively growing express enough of it to be detected by the fluorescence. But that still leaves a good number of clinical applications, including many types of ovarian, cervical, gastric, and colon cancers.

Kobayashi’s team has already begun to evaluate the spray in human tumor samples. It expects to start full clinical trials in a few years.