Intended for healthcare professionals

Research News

Researcher questions new study suggesting benefits of breast screening

BMJ 2018; 363 doi: https://doi.org/10.1136/bmj.k4790 (Published 13 November 2018) Cite this as: BMJ 2018;363:k4790

Re: Researcher questions new study suggesting benefits of breast screening: Researcher’s questions reflect a failure to carefully read the study

Kalager and Jørgensen both fail to realize that no previous publication has measured the incidence of fatal breast cancer. Kalager claims that the methodology we used in our recent paper published in CANCER (1), i.e., the incidence rate of fatal breast cancer” is “really incidence-based mortality”, but she is mistaken. In incidence-based mortality, the X-axis represents the year of death, making correlation with exposure at the time of incidence subject to considerable bias. On the contrary, in our novel method, the X-axis represents the year of diagnosis for breast cancers that become fatal within 10 or 20 years, making possible a direct comparison of screening exposure with non-exposure at the time of breast cancer incidence. As we noted, improvements in therapy do not account for our results because in each comparison year, women received the state-of-the art therapy appropriate for their stage-at-diagnosis, regardless of how their cancer was detected.

Kalager and Jørgensen also raise the issue of healthy screenee bias. Each also failed to notice that our analysis adjusted for selection bias. But it also is the case that healthy screenee bias has little relevance to a service screening program in which more than 85% of all women aged 40-69 participate. How could the entirety of 85% of the population have the super healthy attributes mentioned, as well as being “well-edicated [sic] and affluent?”

We counted the number of women who died of breast cancer within ten years, not “survived” as Jørgensen mistakenly claims. While it also is easy to be dismissive by invoking lead-time bias, he should be aware that most women who are destined to die from breast cancer will have died before 20 years of follow-up. Thus, lead-time bias also is not a concern. Jørgensen states that “the authors do not seem to have taken overdiagnosis into account”. It should be obvious that overdiagnosis is utterly irrelevant when studying fatal cancers, as we pointed out in the Introduction of our article: “Overdiagnosis is not an issue when studying fatal cancers, because an overdiagnosed breast cancer, by definition, cannot ever be fatal.”

Jørgensen’s citation of the IARC Handbook of Cancer Prevention vol. 7 is outdated, as the 2016 update (in which three of us participated) clearly states that “There is sufficient evidence that screening women aged 50–69 years by mammography reduces breast cancer mortality. This evaluation is supported by randomized controlled trials of efficacy of mammography screening and by observational studies of effectiveness of both invitation to and attendance at service mammography screening. Women aged 50–69 years invited to service mammography screening have, on average, a 24% reduced risk of mortality from breast cancer. Women aged 50–69 years who attend service mammography screening have, on average, about a 40% reduced risk of mortality from breast cancer.” (2)

Finally, with respect to his final remarks, as usual, when someone has lost the scientific argument they fall back on accusations of conflict of interest.

References
1 Tabar L, Dean PB, Chen TH, et al. The incidence of fatal breast cancer measures the increased effectiveness of therapy in women participating in mammography screening. Cancer 2019;125:515-23.
2 IARC Working Group on the Evaluation of Cancer-Preventive Strategies. Breast Cancer Screening. Vol 15. Lyon, France: IARC Press; 2016, p 467.

Competing interests: Dr. Tabar reports personal fees from General Electric Healthcare for preparing automated breast ultrasound teaching cases, and honoraria and travel costs to General Electric Healthcare speaking engagements; personal fees from Mammography Education, Inc. in his capacity as president of the company, which organizes CME courses on breast imaging; and personal fees from Three Palm Software for consultation related to breast imaging interpretation, all outside the submitted work. The other authors made no disclosures.

11 February 2019
László Tabár
radiologist
Peter B. Dean, Tony Hsiu-Hsi Chen, Robert A. Smith, and Stephen W. Duffy
Falun Central Hospital
Department of Mammography, Falun Central Hospital, Falun, Sweden