Some cancer patients are being "written off" as being too old for treatment, a charity has warned.
Too many older patients are being assessed on their age alone and not their overall fitness, Macmillan Cancer Support said.
The comments come as new research from the charity and the National Cancer Intelligence Network (NCIN) found that tens of thousands of pensioners who have been diagnosed with cancer have survived for at least a decade.
More than 130,000 people in the UK have survived for at least 10 years after being diagnosed with cancer at the age of 65 or above, the charity said.
This includes more than 8,000 patients who were diagnosed at the age of 80 and over.
But despite the large number of older people who are "long-term" survivors of the disease, the charity said that many patients in the UK are being denied treatment because they are deemed to be too old.
It said that cancer survival rates in this age group are "poor".
For many common cancers - including prostate, breast, lung, stomach, ovary and kidney cancers - the UK and Ireland have a lower five-year survival rate than the rest of Europe, a spokesman said.
Health workers should ensure treatment decisions are not based on age alone and should also assess a patient's physical and mental well-being, the charity said.
"It's wrong to write off older people as too old for treatment," said Macmillan Cancer Support's chief executive Ciaran Devane.
"With a proper assessment and appropriate treatment, our research shows that many older cancer patients can live for a long time and can even be cured.
"While it's good news that so many older people are benefiting from treatment, many thousands more could live longer if our survival rates for over-65s matched those in comparable countries.
"The barriers to getting treatment - which include age discrimination and inadequate assessment methods - must be tackled now so more older people can survive cancer and live for many years."
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NCIN's clinical lead Dr Mick Peake added: "It is vital that all patients receive the best and most effective treatment based on the nature of their cancer and their fitness for treatment and that chronological age alone is not the deciding factor.
"We know that cancer survival rates in older patients in many other countries are better than in the UK and ensuring optimal treatment at all ages is the way of tackling this issue."
Dr Mark Porter, chairman of council at the British Medical Association, said: "It is important that all healthcare professionals ensure that patients are treated on the basis of their clinical need.
"With an increasingly ageing population, it should be a key part of medical professionalism to guarantee that older patients are treated with the care and respect they deserve."
Sean Duffy, national clinical director for cancer at NHS England, said: "In light of the fact that we've got an ageing population, it's a very timely moment to review the way services are aligned around this age group.
"We need to deliver better services for people over 65 and 75 because we know there's an issue and interventions need to be designed to that end.
"Our understanding of cancer continues to improve and patients with cancer now have much more consistent access to high-quality treatment than ever before. This is meaning more and more lives are being saved.
"Treatment for cancer can be complex and should be based on what is right for each individual patient."