The UK and other European countries need to face up to the challenges of tackling this cancer "timebomb", the European Cancer Conference will hear.
Kathy Redmond, the editor of the journal Cancer World, says there are not enough specialist doctors to cope.
The Department of Health has promised a strategy to tackle rising cancer rates.
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.
In her keynote speech to the conference in Barcelona, Kathy 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."
She will call for "current ageist" attitudes to cease.
"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."