By Marilynn Larkin
NEW YORK (Reuters Health) - Lung cancer incidence is higher among young women than men in countries around the world, and the reasons aren't clear, researchers say.
"These findings forewarn of a higher lung cancer burden in women in the decades to follow, especially in higher-income settings," Dr. Miranda Fidler-Benaoudia of Alberta Health Services in Calgary told Reuters Health by email. "Further work is needed to intensify anti-tobacco measures and identify factors for the higher lung cancer burden among young women."
Dr. Fidler-Benaoudia and colleagues studied lung and bronchial cancer incidence rates in 40 countries across four continents. Data were categorized by five-year age groups from ages 30-64, and by five-year calendar periods, from 1993-2012.
As reported in the International Journal of Cancer, among men, age-specific lung cancer incidence rates generally decreased in all countries, while in women the rates varied, remaining stable in most countries or declining, albeit at a slower pace than men.
Therefore, the female-to-male incident rate ratios increased among recent birth cohorts, with IRRs significantly greater than unity in Canada, Denmark, Germany, New Zealand, the Netherlands and the United States.
For example, the IRRs in ages 45-49 years in the Netherlands increased from 0.7 in individuals born around 1948 to 1.5 in individuals born in 1963. Similar patterns, were found in 23 additional countries, although the differences were nonsignificant.
Even among the earlier birth cohorts, there was an indication of higher lung cancer incidence rate ratios in women compared to men for the younger age groups. For example, the IRR in New Zealand among those aged 40-44 increased from 1.0 in the 1953 birth cohort to 1.6 in the 1968 cohort.
These crossovers (from male to female dominance) were largely driven by increasing adenocarcinoma incidence rates in women, the researchers found. Yet, for countries with historical smoking data, the smoking prevalence in women approached, but rarely exceeded, that of men. "This suggests that sex-differences in smoking are unlikely to explain the crossovers observed," Dr. Fidler-Benaoudia said.
"While we can hypothesize why incidence is higher among young women, including increased susceptibility to lung cancer among women compared to men, further work is needed to understand these trends and the changing factors that underlie them," she added.
"We are currently exploring sex-specific lung cancer incidence rates in young blacks versus young whites in the U.S. to determine whether there are still racial disparities, and how trends may relate to smoking patterns by race and sex," she added.
Dr. Joseph Treat, a professor in the Department of Hematology/Oncology at Fox Chase Cancer Center in Philadelphia, commented in an email to Reuters Health that among his patients, "the proportion of lung cancer in never-smokers is increasing and majority of these are women. The causes are still not clear."
"We know that second-hand smoke is a cause," he said, "but this is only a part of the story because many never-smoking patients do not report any significant history of second-hand smoke exposure."
"Given that lung cancer in never-smokers disproportionately affects women, the contribution of hormonal factors and estrogen has been an active area of research and there are studies to suggest that estrogen and how it is metabolized in the body may be a cause, though further research on this is needed," he noted.
"There may be very different contributing factors depending on customs in different countries," he added. "For example, in Asia, it has been suggested that fumes from poorly ventilated cooking oil vapors may contribute to an increase in females developing lung cancer."
SOURCE: http://bit.ly/2uACjI6 International Journal of Cancer, online February 5, 2020.