The study team interpreted patterns of prostate and breast cancer incidence and mortality in four Nordic countries across various age groups from 1975 to 2013. Led by Dr. Rune Kvåle, PhD, from the division for health data and digitalization at the Norwegian Institute of Public Health in Bergen, Norway, the team found the mortality rates for both prostate and breast cancers have decreased steadily over the past two decades (International Journal of Cancer, 10 August 2017).
"Although the relative importance of these measures remains difficult to disentangle, the considerable decrease in breast cancer mortality in the youngest age groups and the early decreases in prostate and breast cancer mortality rates in Denmark below the age of 75, are consistent with favorable contributions from treatment in addition to mammography screening and increased PSA [prostate-specific antigen] testing," they wrote.
The importance of studying trends
How effective is screening? Kvåle and colleagues analyzed incidence and mortality trends by age to better understand the effects of earlier diagnosis and treatment on Nordic mortality rates for prostate and breast cancers since screening was introduced in the early to mid-1990s.
Randomized studies and evaluations of the potential effects of mammography screening and PSA testing concluded approximately 20% to 30% disease-specific mortality reductions for both diseases, with higher estimates of overdiagnosis for prostate cancer than for breast cancer, according to the authors.
"The introduction of a new screening method would be expected to cause increases in incidence most noticeable in those age groups and periods with the largest changes in diagnostic intensity, to be followed, if effective, by reductions in mortality in the corresponding and older age groups," they wrote.
Kvåle and colleagues extracted data from the NORDCAN database for the period 1975 to 2013 to provide an updated description of temporal prostate and breast cancer incidence and mortality trends by age in Denmark, Finland, Norway, and Sweden. They set the age limit five years higher for mortality than for incidence to avoid small mortality numbers in the lowest age category. Also, the researchers calculated the percentage changes between two five-year periods. They selected the period 1980 to 1984 as a baseline for incidence and the period 1985 to 1989 as a baseline for mortality.
Prostate cancer rates
During the late 1970s and the 1980s, the researchers found a parallel increase in prostate cancer incidence rates in all four countries. Around 1990, prompt increases occurred in Norway and Finland, followed approximately five years later by similar increases in Denmark and Sweden. The increase from the early 1980s to the second decade of this millennium represents an approximate doubling of rates in Denmark, Finland, and Norway and a 60% increase in Sweden. More recently, incidence rates in the four countries have either declined (Denmark, Finland, and Sweden) or stabilized (Norway), according to the authors. Incidence rates in Denmark were lower than in the other countries for almost the entire study period.
In terms of mortality rates, at first they increased in the 1980s and 1990s, then they significantly declined annually by 1.9% to 2.7% starting in the mid- to late 1990s in all countries except Denmark, where rates stabilized from about 2000. Total change in mortality from 1985-1989 to 2009-2013 ranged from a 22% decrease in Finland to an 8% increase in Denmark. Prostate cancer mortality rates have stabilized or decreased four to nine years after rises in incidence, declining first in the countries with the earliest increases in incidence, the authors noted.
Decreases in prostate cancer mortality were largest in the youngest age groups: In men younger than 55, mortality declined between 1985-1989 and 2009-2013, ranging from 33% in Finland to 50% in Denmark. In the age group 55-74, the mortality decreases between the same periods ranged from 14% in Denmark to 39% in Norway. However, the same was not true of older men. In men 85 and older, mortality rates in Denmark increased by 1.6% annually in recent years; only Finland experienced any reduction in mortality rates.
Breast cancer rates
Breast cancer incidence increased in all four countries during the study period ranging from a 37% increase in Sweden to 79% in Finland. The most rapid increases were observed from the mid- to late 1980s in Finland and Sweden, from the mid-1990s to the early millennium years in Norway, and from 2007 to 2009 in Denmark.
In Denmark, starting from 2010 on, incidence rates showed a marked decrease. However, incidence rates were higher in Denmark than in the other countries for almost the entire study period.
In terms of the breakdown by age, in the 50-69 age bracket, the increases in breast cancer incidence ranged from 60% in Sweden to more than double in Finland from 1980-1984 to 2009-2013.
In Denmark, Norway, and Sweden, incidence rates in women younger than 50 gradually increased by 0.3% to 0.9% annually throughout the study period. In Finland, incidence rates for women younger than 50 increased relatively rapidly until the mid-1990s before stabilizing.
In Denmark and Finland, moderate increases were observed in the older age group; rates in Norway and Sweden remained relatively stable throughout the study period.
In terms of mortality, in the late 1980s, mortality rates in Denmark were approximately 60% higher than in the other countries, and to this day Danish figures remain 30% to 35% higher, according to the authors. In Sweden, mortality rates gradually decreased over the study period, with a similar decrease in Finland from the late 1980s. From 1985-1989 to 2009-2013, total decrease in mortality ranged from 32% in Norway to 17% in Finland.
When broken down by age, the decline in breast cancer mortality rates was most noticeable in the youngest age groups. In women younger than 55, the decline in mortality between 1985-1989 and 2009-2013 ranged from 33% in Finland (38% in women younger than 50) to 55% in Denmark. In the age group 55-74, declining mortality in the same period ranged from 11% in Finland to 33% in Norway. In the latest years of the study period, the annual decline in mortality ranged from 0.5% in Finland to 9% in Denmark with less favorable trends in older age groups.
Putting it together
When comparing prostate and breast cancers, the data showed mortality rates for prostate cancer declined more than for breast cancer in all countries except Denmark.
"The results are consistent with favorable contributions from advances in treatment, in addition to mammography screening and increased PSA testing," the authors wrote.
Concurrent and continuous increases in both cancer incidence rates, even in periods and countries with limited screening activities, supports the notion the major proportion of the decline in mortality is not driven by changes in risk factors. Only minor changes in risk factors such as alcohol consumption have occurred since the 1990s, yet breast cancer mortality rates have declined, they noted. The cancer risk factors remain nebulous, but early detection and effective treatment play a part, they added.
"Our results, mainly showing that the time lags between introduction of widespread PSA testing and declining mortality conforms to what is expected from randomized studies, the present findings suggest that the recent declining mortality trends can in part be attributed to earlier diagnosis and effective early curative treatment," they wrote.
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