Cancer patients should undergo genetic testing of their tumours in order to boost survival and avoid unnecessary, gruelling treatments, experts say.
Researchers attending the world's biggest cancer conference in Chicago hailed personalised medicine as the "future of cancer treatment" and the "biggest breakthrough since chemotherapy".
While some treatments already hone in on specific biological pathways to stop cancer growing, such as in breast cancer, there is still no widespread use of personalised medicine.
Yet the approach - which is being developed at a rapid pace by the cancer community - means patients could be spared chemotherapy if the genetic profile of their tumour biopsy shows it will be of no benefit to them.
Highly-targeted drugs aimed at bringing the cancer under control can also be selected rather than relying on "guesswork" or a "one size fits all" approach.
Several studies are due to be presented at the American Society of Clinical Oncology (ASCO) conference showing the benefits of personalised medicine.
One trial of more than 13,000 patients found targeting therapies dramatically boosted the numbers who saw their tumours shrink compared to standard treatment.
In another, experts from the University of Cambridge Hospitals and University College London (UCL) believe thousands of women every year could be spared chemotherapy if their genetic test shows their tumour will not respond to it.
Rowena Sharpe, head of precision medicine at Cancer Research UK, said personalised medicine "is a very, very different way of treating patients".
She added: "It's the most exciting thing since chemotherapy.
"It's about giving the patient the most appropriate choice. We don't want to treat them with a drug that isn't going to do anything."
Professor Roy Herbst, chief of medical oncology at Yale Cancer Centre in the US, said precision medicine was "about finding the right key for the lock, finding out what it is that is driving the tumour, what makes it tick".
He added: "At the moment, it is informed guesswork, so that treatment often doesn't work for large numbers of patients.
"I believe the potential (of personalised medicine) is huge."
Professor Johann de Bono, head of drug development at the Institute of Cancer Research, London, and the Royal Marsden NHS Foundation Trust, agreed it was an "exciting time in the field" with "major benefits for patients".
He added: "What patients need to be aware of, is that we can now look at their tumour by genomic testing and if we identify what we call, actionable genomic aberrations, that we can use that to decide what drug to give the patient.
"It's no longer just thinking about whether the cancer has come from the breast or the bowel or the lung. There's more to it than that."
Prof de Bono said the practice was still not used widely on the NHS as there is still a large amount of research to do.
He said the tumour testing involved extracting DNA from biopsies and putting it through a sequencing machine.
The cost of tests can vary from £200 for targeted sequencing of a couple of genes, up to around £3,000 for a more comprehensive analysis.
Prof de Bono stressed things were "changing very quickly" and personalised medicine was already having an impact for some cancers including prostate, ovarian, breast, lung and bowel.
The University of California, San Diego School of Medicine study presented at ASCO, which pooled the results of 346 early-stage clinical trials involving more than 13,000 patients, has shown the benefits of personalised medicine.
Researchers found that tumours in patients who received targeted treatments had shrinkage rates of 30.6%, compared with 4.9% in those who did not.
Those on targeted treatment also lived longer without their disease progressing.
Dr Robert Stein, a consultant in breast cancer at UCL, is also launching a trial on genetic testing for 4,500 women newly-diagnosed with breast cancer.
It could see thousands of women a year avoiding chemotherapy, saving the NHS millions of pounds.
He said: "In every area of cancer treatment, we have largely functioned on a one size fits all basis, because we didn't have tools to do any better.
"What personalised medicine does is identifies groups of patients who are not likely to benefit from a particular treatment.
"We would expect to reduce chemotherapy within the trial population by about two thirds.
"We are looking at between 5,500 and 7,000 fewer a year being treated with chemotherapy than are currently treated."
Baroness Delyth Morgan, chief executive of Breast Cancer Now, said: "Personalised medicine represents the future of cancer treatment."