Using a scanner rather than a scalpel could spare hundreds of thousands of cancer patients from risky surgery, a study suggests.
Head and neck tumours are treated with chemotherapy and radiotherapy, but then need an operation to visually check whether the growth has gone.
A study on 564 patients, published in the New England Journal of Medicine, showed 80% of them could be spared surgery by scanning instead.
And survival rates stayed the same.
The operation to check a cancer in the head or neck takes three hours, and patients spend at least a week in hospital recovering.
It also risks complications, including disfigurement or movement problems in the arms if key nerves are damaged.
Positron emission tomography-computed tomography (PET-CT) uses a radioactive dye that is picked up by rapidly dividing cancer cells.
This allows doctors to see if any of the head or neck cancer is still active.
Prof Hisham Mehanna, from the University of Birmingham, told the BBC News website: "Cancerous cells hide among the dead cells, with PET-CT you can call them out and find out whether they're alive or not.
"We can now use this new technology to save patients having a debilitating operation and identify those that need the operation rather than give it to everybody."
The study, conducted by the Universities of Birmingham and Warwick, showed survival rates were the same with both the surgical and the scanning approaches.
But only one in five of the patients had actually needed an operation to remove cancerous tissue.
Prof Mehanna said scanning could help hundreds of thousands of people around the world each year.
Using the PET-CT scan approach also saved the health service £1,492 per patient.
Life Sciences Minister George Freeman said: "This exciting trial has the potential to make a real difference to the lives of people with head and neck cancer, meaning that they may not have to undergo an extremely stressful medical procedure."
Prof Arnie Purushotham, from Cancer Research UK, said: "This is a really important study and if long-term follow-up confirms these results, this imaging technique could mean kinder treatments for patients with head and neck cancer.
"There could also be opportunities to expand this approach to other types of cancer and also potentially saving money for the NHS."