Stephen Duffy and László Tabár, both long-standing proponents of the benefits of mammographic screening, studied over half a million Swedish women, which included almost a third of the population eligible for screening. Their team – there were over 30 co-authors – utilised a novel endpoint: the frequency of breast cancer becoming fatal within 10 and 20 years after diagnosis (covering the years 1992 to 2016). This meant that exposure to screening, breast cancer treatment and death from breast cancer all belonged to the same time period.
Since all women – whether they were screened or not – received treatment appropriate for their disease stage, Duffy and colleagues argued that any changes seen could not be accounted for by improvements in treatment. Plotted at left (© American Cancer Society) is the cumulative incidence of breast cancer that was fatal within 10 years after diagnosis vs the year of diagnosis. As Tabár commented, “The results speak for themselves. The 41% decrease is above and beyond any effect modern therapeutic regimes might have had.” It all comes down to screening and early detection.
A different perspective on the utility of screening has been provided by Nora Pashayan and Paul Pharoah in a brief commentary in Science entitled “The challenge of early detection in cancer.” Their primary insight is captured in the subheading, “Tumour growth dynamics and the timing of metastasis impose limits on cancer screening,” and the diagram at right (© AAAS).
Mammographic screening can detect tumours that are 0.5 cm in diameter, while clinically detectable tumours are at least 2 cm in diameter. For breast cancer, a typical tumour volume doubling time (TVDT) is 150 days and it will have been increasing in size for 8 years before it is big enough to metastasize. However, another 3 years will pass before the lesion is large enough for detection by mammography. That said, it is hard to ignore the evidence provided by Duffy and his co-authors, so there appears to be little doubt that early detection does indeed save lives.