Re: Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial
It appears that the authors conclusions that annual mammography does not reduce mortality in women age 40-59 beyond that of physical examination or usual care oversteps the limitations of the study. The correct conclusion is that in the 1980s annual clinical breast examinations and training in self breast exam produced nearly equivalent breast cancer specific mortality rates compared to annual mammography screening coupled with annual clinical breast exams and training in self breast exams.
The reasons are the following:
1) All participants in the randomized clinical trial (RCT) had clinical breast exams and were taught breast self examinations at the beginning of the study. The women 40-49 years of age were thereafter randomized to mammography or usual care but the 50-59 year old women were randomized to mammography and annual physical examination or no mammography and annual physical examination. At no point in the RCT was there an arm without either initial clinical breast exam, training in self breast exam, or usual care.
2) A relatively small number of cancers, 32%, were found by mammography alone in the mammography arm of the study. The rest were palpable and were found by either the patient or at the annual physical exam.
3) The mean tumor sizes found in the control arm are not what would be expected in a study with a nonintervention control arm. In our study of mammography detected breast cancer among women age 40-49 we found mean size of mammography detected tumors to be 1.8 cm and the patient/physician detected (palpable) tumor mean size was 3 cm. (1)
4) There were 10% fewer deaths from breast cancer among the mammography detected tumors (19.8%) compared to the palpable tumors (30.4%) in the mammography arm of the study indicating an advantage from detection of breast cancer at an earlier non-palpable stage.
The author’s assertions of overdiagnosis are hard to accept at face value when even the patients with the smallest non-palpable tumors found in the mammography screening arm had a 20% death rate from breast cancer. In our study of breast cancer mortality and treatment by detection method with cases from 1990 to 2008 tracked by our institutional cohort registry the observed death rate from breast cancer among mammography detected cancers was 4% and among patient or physician detected breast cancers was 11%.(1) All breast cancer discovered in this RCT had a hefty breast cancer death rate of 20 to 33% presenting a situation in which it does not appear any of the breast cancer discovered was not going to present harm to the patient during her lifetime. The high rate of breast cancer specific mortality seen in all arms of the study may be due to treatment that has currently been improved upon but was standard of care in the 1980s.
It is not clear if the technology in use in the 1980s when this particular RCT was conducted are comparable to current technology and is particularly questionable given the small difference in mean tumor size observed between arms. Given the small differences in size observed it is doubtful this study has relevance to current technology and the capacity of mammography to be superior to palpation as a method to detect breast cancer at an earlier more treatable stage.
1) Malmgren JA, Parikh J, Atwood MK, Kaplan HG. Impact of mammography detection on the course of breast cancer in women aged 40-49 years. Radiology 2012;262(3):797-806.