‘As epidemiologists, we are not alarmed by the apparent ‘increase’ in deaths from prostate cancer,’ here’s why

Jason Oke

For the first time in the UK, the number of deaths from prostate cancer has exceeded the numbers of deaths from breast cancer. According to figures from the UK’s Office for National Statistics (ONS), more men died of prostate cancer (11,819) than women died from breast cancer (11,442).

The Guardian, like many media outlets, pointed to the changing age demographics as a likely contributor; prostate cancer is a disease of the elderly and men are living longer. Others, however, commandeered the media attention to push forward their own hypotheses. Roger Wotton, chairman of Tackle Prostate Cancer, said: “Women have screening for breast cancer and this is one reason why mortality rates for prostate cancer are now higher than those for breast cancer”. As epidemiologists, we are not alarmed by the apparent ‘increase’ in deaths from prostate cancer. In this blog, we outline why.

Commentators have reported that, over time, breast cancer prognosis has improved at a greater rate than prostate cancer. However, this conclusion is based on data that doesn’t take the change in population demographics over time into account. To do this we need to use age-standardised mortality rates rather than “crude” numbers of total mortality^1,2. Standardised rates allow for a fair comparison over time as it removes the confounding effect of age³. The figure below shows prostate and breast cancer mortality rates standardised to a reference population. What is clear is that both prostate cancer and breast cancer deaths have been declining since the early nineties, and prostate cancer standardised mortality has been higher than breast cancer for the last 28 years – hardly news at all.

Some clarity can be found by examining the mortality figures stratified by age. The death rate from prostate cancer in men older than 80 years of age is 601 per 100K, whereas the breast cancer mortality rate for women aged 80 or older is 216 per 100K.

In the year 2000, there were 724K men and 1,485,600 women aged 80 or over. Women have historically had a higher life expectancy, so this is not surprising. However, some of this shortfall may be a legacy of War. For a man to have been >80 in 2000 he would have been born before 1920 and eligible to be in the 2nd World War. Fast-forward to 2015 and the difference between men and women older than 80 is much narrower; the number of women >80 increased by 19%, compared with 66% in men. This increase in elderly population for both sexes translates to 663 extra breast cancer deaths but 3000 extra prostate cancer deaths, contributing to the increasing crude number of prostate cancer deaths.

The assessment of trends over time should always be made using age- and sex-standardised rates. Without adjusting for differences in population across years, crude numbers can prove to be misleading.

Jason Oke is a Senior Statistician in the Nuffled Dept of Primary Care Health Sciences and Module Coordinator: Introduction to Statistics for Health Care Research on the  Programme in Evidence-Based Health Care at the University of Oxford. 

Acknowledgements; thank you to Jack O’Sullivan and Brain Nicholson for helpful comments on this piece.

Conflicts of interest: none declared

References

1. Cancer Research UK. Prostate Cancer Mortality Statistics. Available at: http://www.cancerresearchuk.org/health-professional/cancer-statistics/statistics-by-cancer-type/prostate-cancer/mortality. (Accessed: 5th February 2018)
2. Cancer Research UK. Breast Cancer Mortality Statistics. Available at: http://www.cancerresearchuk.org/health-professional/cancer-statistics/statistics-by-cancer-type/breast-cancer/mortality. (Accessed: 5th February 2018)
3. Kirkwood, B. & Sterne, J. Essential Medical Statistics. in (Wiley-Blackwell, 2010).