People aged over 75 years accounted for one-third of all cancer deaths in 1981, and by 2001 half of all cancer deaths were among this group, notes Kathy Redmond, editor of the journal Cancer World.
One thing is certain - there will be many more elderly people living with cancer. There is still far too much complacency about this time-bomb, she will say in her her keynote speech to the European Cancer conference in Barcelona.
Redmond, herself a nurse, will suggest this means specialists will find themselves not treating just cancer in the future, but, increasingly, treating cancer alongside a catalogue of other serious age-related medical problems.
She wants to see more specialists ready to deal with the added complexity of helping patients who may also be suffering from dementia or movement problems.
"There is an inverse relationship between increasing age and the likelihood of proper treatment despite evidence that otherwise healthy elderly cancer patients can benefit from treatment to the same degree as their younger counterparts," she will say.
"Under-treatment and sub-optimal practices mean that older patients are dying unnecessarily from cancer."
"It is almost impossible to predict what the reality will be in 2020," she will argue.
"But one thing is certain - there will be many more elderly people living with cancer. There is still far too much complacency about this timebomb."
One of the biggest problems for cancer care, she says, is patients who do not comply with complex treatment regimes, often involving more than one drug.
This is a particular problem in the elderly, who are more likely to be forgetful or lack the social support they need to stay organised.
"There is a need to train health professionals how best to promote treatment adherence and to recognise when patients are not complying with their treatment," she says.
"Policy makers need to plan and invest today to ensure that there will be an adequate infrastructure in place to care for the vastly increased numbers of cancer patients in 2020."
UK's Department of Health said a strategy for England would be published later this year.
A spokesman said: "Cancer is predominantly a disease of the elderly and the fact that people are living longer may be one reason for the increase in incidence rates.
"The Cancer Reform Strategy will take account of this increasing incidence when it sets out the future for cancer services.
"Overcoming inequalities in cancer services, including age issues, has also been considered as part the strategy."
Richard Davidson, Cancer Research UK's director of policy and public affairs, said: "Countries must build effective cancer strategies if they are to prepare properly for the future challenges of an ageing population and the increasing costs of cancer treatments.
"It is projected that the number people over 65 will double in the next 15 years in the UK and the price of treatments looks set to continue to rise."