People respond differently to chemotherapy
A sophisticated scanning technique has allowed researchers to watch in real time as anti-cancer drugs attack a patient's tumour.
The technique should allow doctors to make a more accurate assessment of the effectiveness of individual cancer drugs.
It should also enable them to spot early signs that treatment is failing to work, allowing a speedy switch to an alternative drug.
And it may also dramatically speed up clinical trials, by making it far easier for researchers to collect information on a drug's function.
This technique will greatly assist the assessment of new anti-cancer drugs
At present, it is very difficult for doctors to assess whether a cancer treatment is succeeding without undertaking an invasive biopsy.
But researchers based at the MRC Cyclotron Unit in London's Hammersmith Hospital and the Northern Institute for Cancer Research in Newcastle used a high tech, non-invasive imaging technique called Positron Emission Tomography (PET) to track the effectiveness of treatment.
Scientists studied patients who were being treated at Newcastle General Hospital for bowel or stomach cancer, flying them down to the Hammersmith for their PET scans.
The patients were receiving a drug called nolatrexed (or Thimitaq), which stops cancer cells from making thymidine - a key component of DNA that is vital for cell division.
By injecting them with mildly radioactive molecules of thymidine, researchers were able to watch on screen as the anti-cancer drug took effect.
Researcher Professor Pat Price said: "When a drug prevents tumours from making the molecules they need for their growth, they have to frantically smuggle them in from outside instead, and we can visualise this with the new scanning technique to build up a picture of exactly how well the treatment is working."
Cancer cells only draw in thymidine from outside if a drug is preventing them from making it, so such an action is a marker of the effectiveness of treatment.
But if cancer cells are able to smuggle in very large quantities of thymidine, it may give them resistance to the drug's effects, and once perfected, the new system should be able to detect this as well.
Professor Price said: "It should allow us to make much earlier decisions about whether or not a drug is working, opening the way to a far more flexible approach to treating patients, with doctors able to turn from one drug to the next as necessary."
Professor Robert Souhami, Cancer Research UK's director of clinical research, said: "For the first time we have a technique which allows us to measure directly within a tumour whether a drug is working, without the need for a biopsy.
"This technique will greatly assist the assessment of new anti-cancer drugs and allow them to get into clinical practice more quickly."
Details of the technique are published in the Journal of the National Cancer Institute.