Cancer prevalence rates for most types of cancer are on the rise in Canada, Statistics Canada reports, in part because the population is aging and survival rates are increasing.

In its first detailed report on cancer prevalence trends in Canada, the agency reports that the five-year prevalence rate for cancer rose 2.1 per cent a year between 1997 and 2008.

It should be noted that cancer "prevalence" is different from cancer "incidence," which refers to newly diagnosed cases. Prevalence includes all the people living after a diagnosis.

So changes in prevalence rates can reflect not just changes in the number of people being newly diagnosed, but also the number of people surviving.

"Several factors, the importance of which varies by cancer type, may account for changes in incidence and survival," the agency notes.

Aging population mostly to blame

The agency estimates that roughly half of the increase in five-year cancer prevalence seen during the study can be chalked up to the fact that Canada's population is aging, because the risk of most cancers increases with age.

An aging population is why, for example, the prevalence rates for prostate cancer rose substantially during the study,

Prostate cancer is the most common cancer in Canada. The study found that the five-year prevalence rate for this cancer increased 3 per cent per year from 1997 to 2008.

But increases in the prevalence of breast cancer -- the most common cancer in women -- were more moderate. Between 1997 and 2008, the five-year prevalence rate rose by an average of 1.3 per cent per year.

Liver and thyroid prevalence up

The prevalence for both liver and thyroid cancer went up significantly during the study period. In fact, the average annual increases in five-year prevalence rates for both cancer types were more than double the increase for any other cancer.

And yet the reasons for the increases in the prevalence in these cancers are likely quite different.

The five-year liver cancer prevalence rate rose 8.3 per cent per year, with the increases higher in men. That rising rate most likely due to an increase in new diagnoses, since the five-year survival rate for liver cancer is low, says Heather Bryant, the vice-president for cancer control at Canadian Partnership Against Cancer.

"The survival is getting a bit better for liver cancer, but it's one of those cancers, like lung cancer and pancreatic cancer, where, unfortunately, the outlook isn't good," Bryant told CTV News Channel Wednesday.

With thyroid cancer, on the other hand, the prospects for treatment are quite good, so the increase in its five-year prevalence is likely due to survival, rather than an increase in incidence.

"The survival rate for thyroid cancer is really very good; it's about 98 per cent. And so anyone who gets diagnosed with it becomes part of the prevalence statistic," Bryant said.

Lung cancer gender differences

For lung cancer, the five-year prevalence rate increased 2.6 per cent per year since the beginning of 2005, up from a rate of less than 1 per cent a year before this period.

In men, the prevalence rate dropped slightly, 0.3 per cent per year. But in women, it increased 3.0 per cent per year.

"This discrepancy was the result of sharper decreases in smoking prevalence among men since the mid-1960s," the agency said.

The only cancer types that saw declines in prevalence during the study period were cervical cancer and cancer of the larynx.