A "relatively large" increase in the prevalence of liver cancer, which is nearly 100 times less common than prostate cancer, has been reported by Statistics Canada.
The agency released its report, Canadian Trends in Cancer Prevalence, on Tuesday, calling it the first such detailed report of the trends in the country.
Prevalence rates for prostate cancer, the most common cancer in Canada, rose substantially, mainly because of the aging of the population over the study period 1997 to 2008, according to the report.
Increases in the prevalence of breast cancer, the second most common cancer and the most common in women, were "more moderate."
Prevalence is defined as all cancers diagnosed within a given period among people alive on a specified date. In contrast, "incidence" refers to newly occurring cases.
The five-year prevalence rate for cancer overall rose 2.1 per cent a year between 1997 and 2008.
Liver cancer factors
Larry Ellison and Kathryn Wilkins of the agency’s health statistics division said both the incidence and observed survival rose over the study period, with only about 20 per cent of the increase in prevalence due to aging of the population.
Various explanations for rising liver incidence have been suggested, they said, including:
- Increases in immigrants from countries where hepatitis B and C virus infections and exposure to aflatoxins that can lead to liver cancer are more common.
- Rising incidence of hepatitis C infection linked to intravenous drug use and sharing of needles.
- Growing rates of obesity and Type 2 diabetes.
Liver cancer was the least prevalent cancer studied, with a five-year prevalence proportion of 6.2 cases per 100,000 persons on Jan. 1, 2008. For perspective, the corresponding figure for prostate cancer was nearly 100-fold higher at 610 cases per 100,000 men, Statistics Canada noted.
Many countries have also reported increases in thyroid cancer incidence rates, especially among young and middle-age women. Advances in diagnostic techniques are thought to be a factor, though a recent U.S. study suggested more detection alone can't explain the increase in that country, the authors said.
Radiological tests such as ultrasound and CT are picking up smaller lumps in the thyroid that aren't visible in clinical exams and some of turn out to be malignant, said Dr. Ali Imran, director of the thyroid oncology and neuropituitary clinics at Dalhousie University in Halifax.
"Others have suggested a higher radiation exposure but that wouldn't explain why it is commoner in women," he added in an email.
Rates declined for cancers of the larynx and a type of uterine cancer.
The biggest disparity between the sexes was for lung cancer. The five-year prevalence proportion fell slightly among men but rose among women — a difference that was attributed to sharper decreases in smoking prevalence among men since the mid-1960s.