Scientists have developed a pill that makes tumours light up when exposed to infrared light, paving the way for a breast cancer diagnosis that is more effective than mammograms.

Mammography is an imprecise tool. About a third of breast cancer patients treated with surgery or chemotherapy have tumours that are benign or so slow-growing that they would never have become life-threatening, researchers said. In other women, dense breast tissue hides the presence of lumps and results in deaths from treatable cancers. Mammograms are also notoriously uncomfortable. "We overspend USD 4 billion per year on the diagnosis and treatment of cancers that women would never die from," said Greg Thurber, an assistant professor at University of Michigan in the US.

"If we go to molecular imaging, we can see which tumours need to be treated," said Thurber, who led the team. The move could also catch cancers that would have gone undetected. Researchers used a dye that responds to infrared light to tag a molecule commonly found on tumour cells, in the blood vessels that feed tumours and in inflamed tissue. By providing specific information on the types of molecules on the surface of the tumour cells, physicians can better distinguish a malignant cancer from a benign tumour.

Compared to visible light, infrared light penetrates the body easily - it can get to all depths of the breast without an X-ray's tiny risk of disrupting DNA and seeding a new tumour. Using a dye delivered orally rather than directly into a vein also improves the safety of screening, as a few patients in 10,000 can have severe reactions to intravenous dyes. These small risks turn out to be significant when tens of millions of women are screened every year in the US alone.

Researchers have demonstrated that the concept works in mice. However, it is not easy to design a pill that can carry the dye to the tumour. "To get a molecule absorbed into the bloodstream, it needs to be small and greasy. But an imaging agent needs to be larger and water-soluble. So you need exact opposite properties," Thurber said. Researchers repurposed a drug that had earlier proved to be ineffective in treating cancer and related diseases.

"It's actually based on a failed drug. It binds to the target, but it doesn't do anything, which makes it perfect for imaging," Thurber said. The targeting molecule has already been shown to make it through the stomach unscathed, and the liver also gives it a pass, so it can travel through the bloodstream. The team attached a molecule that fluoresces when it is struck with infrared light to this drug. Then, they gave the drug to mice that had breast cancer, and they saw the tumours light up.