A quarter of long-term cancer survivors can suffer ongoing symptoms of fatigue, insomnia or pain which are side effects of treatment.
Prof Patricia Ganz of UCLA School of Public Health in California told a conference, attended by hundreds of Irish cancer survivors, that others can suffer heart failure, lung problems or a slowing of an arm or a leg which can "surprise people" several years later.
"If a woman has radiation to the chest for lymphoma in her twenties she could get breast cancer later or a narrowing of the coronary arteries.
"If the chemotherapy given injured the heart it could cause heart failure.
"In that one patient you could have a surprise series of events 15 or 20 years later," she told the two-day conference organised by the Irish Cancer Society.
"Some people will have ongoing symptoms. They can be told it is in their head or they are lucky to be alive. They really are suffering and we need to take them seriously."
Speaking to 'Health and Living' she said other potential symptoms include a form of "chemo brain", where someone who had the therapy can suffer a form of mental cloudiness.
It tends to be associated with intensive treatment such as bone marrow transplant or high dose chemotherapy.
It can affect about 20pc of women who had breast cancer.
"They look fine and the cancer has gone away but they are left with these difficulties. Doctors vary in the level of awareness they have of the symptoms.
"Once we understand what is causing the problems we can either try to modify our therapies a little bit or find ways to control or reduce the side effects.
"We tend to be very aggressive with our treatment, but there now is more refinement or tailoring of therapies.
"It would help to identify people early on."
She said there was a lot of evidence that regular exercise and yoga can improve fatigue. People are also looking at different drugs.
"It may be that if we had people exercising during their treatment rather than taking it easy when tired we might be able to prevent this but that has not been well studied," she pointed out.
There is an increasing recognition that there should be a post-treatment assessment and care plan as standard for everyone who finishes treatment.
There is a need for more information about the possible consequences of cancer and its treatment, as well as advice and support on lifestyle issues and how to manage the long-term consequences of their condition.
It comes against a background showing that over 60pc of people in Ireland now survive cancer for five years or longer.
Olwyn Ryan, patient support services manager at the Irish Cancer Society said: "While cancer survival rates are improving, we need to recognise the issues which cancer patients can be left with after the illness itself has gone."