Young women in Canada were more likely to get breast cancer in recent years than three decades ago, according to a new analysis that shows the rates increased most among patients in their 20s and 30s.
The study, published Friday in the Canadian Association of Radiologists Journal, found that young women are still much less likely to develop breast cancer than senior citizens, but their risk is increasing in a way that mirrors findings about early onset cancer in other countries and other parts of the body.
“Twenty years ago, we saw women with breast cancer in their 20s so rarely that it would be something we would show in rounds,” said Jean Seely, head of breast imaging at the Ottawa Hospital and one of the study’s authors. “Now we’re seeing it on a regular basis.”
The rising rates of breast cancer among Canadian women in their 20s and 30s – when many are in the prime of life, completing post-secondary education, raising young children and scaling the career ladder – is part of a larger trend toward increasing incidence of early-onset cancers in developed countries the world over.
Early-onset cancer, which is usually defined as cancer in adults younger than 50, is becoming more common in at least 14 different types of cancer, including breast, colorectal and pancreatic, according to a 2022 study in the journal Nature that asked, “Is early-onset cancer an emerging global epidemic?”
A separate study in BMJ Oncology found that, globally, incidence rates of early-onset cancer increased by nearly 80 per cent between 1990 and 2019.
“It’s a mystery,” said Shuji Ogino, a professor of pathology at Harvard Medical School and one of the authors of the Nature paper. “But we know many risk factors already for adult cancer. We have good hypotheses.”
Topping the list of suspects is what Dr. Ogino calls the “modernizing lifestyle,” including an obesity-promoting Western diet laden with processed foods. Other suspects include a lack of exercise, increased alcohol consumption, modern sleep patterns, stress, and exposure to chemicals and pollutants.
The new paper drew on Statistics Canada population and cancer registry data to analyze breast cancer incidence in Canada over time, broken down by age group.
The researchers found that among those in their 20s, there were, on average, 5.7 cases per 100,000 people of breast cancer annually between 2015 and 2019, up from 3.9 cases per 100,000 between 1984 and 1988 – a 45.5-per-cent increase.
For 30-somethings, there was a 12.5-per-cent increase over the same time span. The rate rose to 42.4 cases per 100,000 in the 2015-2019 period from 37.7 cases per 100,000 in 1984-1988.
Rates also increased for some groups of older women, including those in their 40s, but at slower rates than for their younger peers. Fortunately, overall breast cancer mortality rates have fallen significantly during the same period in Canada as treatments improved and organized screening mammogram programs, primarily aimed at women 50 to 74, detected cancer earlier in older women.
When it comes to breast cancer, another factor could be the trend of delaying or forgoing childbearing in wealthy countries, including in Canada. The new paper notes that the average age at which Canadian women give birth rose to 31.6 in 2022, up from 23.5 in 1966. Hormone changes that occur during pregnancy and breastfeeding are believed to lower risk for breast cancer.
Anna Wilkinson, a GP-oncologist at the Ottawa Hospital and another author of the new study, suspects a “combination of factors” are behind the rising incidence of breast cancer in young women.
“We know things like alcohol exposure and physical inactivity can increase your risk,” she said. “We see that women are having their first child at a later age and they’re not having as many children. So those protective factors, like progesterone exposure during pregnancy and breastfeeding, are lessened.”
One factor that isn’t contributing to the increase is screening. That’s because regular screening mammograms are not recommended for average-risk women in their 20s and 30s in Canada.
Dr. Seely and Dr. Wilkinson, who have been vocal advocates of screening for breast cancer starting at age 40, say the disease remains rare enough in women below 40 that widespread screening wouldn’t be a sensible use of health-care resources. However, they urged younger women who notice changes to their breasts, including discolouration or lumps, to get their symptoms checked out.
In 2019, the most recent year included in the paper, the breast-cancer incidence rate was 140.76 cases per 100,000 for Canadian women in their 40s, 42.47 per 100,000 for those in their 30s and 6.07 for women in their 20s.
The Canadian Task Force on Preventive Health Care, which issues national recommendations to primary-care providers, currently recommends against screening mammograms for average-risk women age 40-49, saying the risks of false positives and overtreatment outweigh the benefits for that age group. The task force is due to publish a highly anticipated update to its guidelines this spring.
Kelly Grant