An Article Critique - Mammography Screening for Breast Cancer
Yao Luo
University of San Francisco

BACKGROUND Despite the high incidence rates, in Western countries, 89% of women diagnosed with breast cancer are still alive 5 years after their diagnosis, which is due to detection and treatment (Parkin, D., & Pisani, 1999). Mammography is such a widespread screening in most developed countries, with the aim of reducing breast cancer mortality through early detection of the disease. However, the organization and delivery vary across geographic regions in ways that may influence its effectiveness (Domingo et al., 2015). In this article critique paper, I choose two relevant peer-reviewed articles using case-control study design to explore the effect of mammography screening on breast cancer survival. The first one is Mammography Screening and Risk of Breast Cancer Death: A Population-Based Case–Control Study (Suzie et al., 2011).
STUDY AIMS The authors conducted a case–control study to assess the effectiveness of the Dutch population–based program of mammography screening. The hypothesis of this study was that mammography screening was associated with a decreased risk of dying from breast cancer.
METHODS
A. What was the main outcome of interest? The main outcome was the breast cancer. Data on breast cancer were obtained from the Comprehensive Cancer Center Rotterdam. Causes of death were obtained through linkage with Statistics Netherlands.
B. What was the main explanatory variable of interest? Mammography screening was the main explanatory variable of interest. It was measured by the National Supervisory Committee of Population Cancer Screening Registry system.
C. What type of study design was this? This was a case–control study. The study design was appropriate because, in order to evaluate the efficiency of mammography screening individually, data on screening history, breast cancer diagnosis, and cause of death were required. So a case-control study can be conducted with these kinds of data.
D. Describe the study population. The sources were 375068 of women who were invited to the mammography screening program in the period 1990 to 2003 in the southwest region of the Netherlands, who did not object to registration or exchange of their records, and were either still alive or died of any cause between 1995 and 2003. In the selection process, 4980 women were excluded. Then, the sample size of cases were 755 women who died from breast cancer and were closely matched to five controls (n=3739) on year of birth, year of first invitation, and number of invitations before case’s diagnosis.
E. What type of analysis did they do? Odds rations and T-stage distribution were used as the measures of association. The STATA: version 9.2, Stata Crop was used for Statistical tests. And the 95% confidence intervals (logistic regression analyzes were used) were reported.
F. Possible bias? In this study selection bias occurred, and the authors used other data to replace the data they can not access. This replacement could skew results directly, but the magnitude was difficult to predict. However, the authors used the Duffy and colleagues method to correct the relevant ORs, thus to minimize the possible self-selection bias.
G. What were the main results? Among the cases, 29.8% was screen-detected, 34.3% interval-detected and 35.9% never-screened. About 29.5% of the never-screened cases had stage IV tumor compared with 5.3% of the screen-detected and 15.1% of the interval-detected cases. The ORs corresponding with screening within the index period and screening following the index invitation yielded an estimate of 0.45 (95% CI, 0.37–0.54) and 0.48 (95% CI, 0.41–0.58), respectively. The odds ratio<1 indicates reduced odds of disease with exposure to the risk factor, in other words, mammography screening has the protective effect of breast cancer.
H. What were some of the study limitations & strength? The authors have mentioned the potential limitation of this study was the self-selection bias. Another limitation was opportunistic mammography screening that was not registered in databases of the screening organizations. The strength was they included a large number of case and control subjects that were derived from the same and well-defined cohort of women to correct the drawbacks. The adopted selection criteria enabled equal opportunity of exposure to screening for both cases and controls, as matching was not only based on year of birth and index invitation but also on the year of first invitation, same age, and number of invitations up to and including the index invitation.
I. What were the author’s main conclusions? The authors conclude that early detection by mammography screening had a beneficial effect in reducing the risk of breast cancer death among women invited to and who attended the screening. Although bias was present, this conclusion was justified by the numerous findings. The second paper is Breast cancer mortality in relation to receipt of screening mammography: a case control study in Saskatchewan, Canada (Pocobelli & Weiss, 2015).
STUDY AIMS The aim of this paper was to evaluate the efficacy of screening mammography in Saskatchewan, Canada. The hypothesis of this study was that mammography screening was associated with a decreased risk of breast cancer.
METHODS
A. What was the main outcome of interest? The main outcome was the breast cancer. It was identified from the vital statistics death registry of Saskatchewan and the Saskatchewan Cancer Agency’s cancer registry
B. What was the main explanatory variable of interest? The main outcome was the breast cancer. It was identified from the vital statistics death registry of Saskatchewan and the Saskatchewan Cancer Agency’s cancer registry
C. What type of study design was this? This was a case–control study.
D. Describe the study population. The source of cases was 1565 breast cancer patients, who died of breast cancer at 50–79 years of age during 1990–2008 and who had continuous Saskatchewan healthcare coverage for at least 5 years prior to their first primary breast cancer diagnosis were identified from the vital statistics death registry. In the selection process, some women were excluded. The final sample sizes of cases were 501 women. The samples of 5009 controls were enumerated from the population registry after excluding women not eligible for the Drug Plan. Controls were randomly sampled and matched to cases on birth year and duration of healthcare coverage prior to the cases’ breast cancer diagnosis date.
E. What type of analysis did they do? Odds ratios (ORs) and 95% confidence intervals (logistic regression analyzes) were used for measuring the association between receipt of a screening mammogram during the 2-year (or 3-year) period prior to the index date and risk of death from breast cancer.
F. Possible bias? In a case-control study, selection bias is general. But in this study, the selection bias was well controlled by the authors. Because all of the eligible cases, identified from the cancer registry and vital statistics death registry, and all sampled eligible controls, identified from the population registry were included. Besides, there was no recall bias. Because the screening history prior to the index date was ascertained using prospectively recorded data, and therefore, it was not subject to errors in recall.
G. What were the main results? The main results were receipt of a screening mammogram in the preceding 2 years was more common among controls (53 %) than cases (37 %), OR 0.51 (95 % CI 0.42–0.62). A decreased risk was observed among women in all age groups, including those 70–79 years (OR 0.40; 95 % CI 0.27–0.60). The odds ratio<1 indicates reduced odds of disease with exposure to the risk factor, in other words, mammography screening has the protective effect of breast cancer.
H. What were some of the study limitations & strength? The limitation mentioned in the paper was this study may underestimate the efficacy of screening mammography in reducing breast cancer mortality. Because accidentally screening in women with symptoms and high possibility to screening in women with a family history would falsely increase the ORs. Besides, the authors artificial set up a threshold to ignore the potential confounders that would affect ORs that underestimate any true benefit of screening mammography. The strength of this study was that possible bias (selection bias and recall bias) were well controlled by the design.
I. What were the author’s main conclusions? The authors conclude that receipt of screening mammography among women in Saskatchewan has been associated with a decreased risk of death from breast cancer. A decreased risk was present not only among women in their 50s and 60s, but also among women in their 70s.
IMPLICATIONS
A. Can the results be generalized to a larger population? Yes. Because in these two studies, the mammography screening was the population-based national program. These programs include an administrative structure responsible for implementation, quality assurance and evaluation of the entire screening process that includes information and invitation of the eligible women or performing the screening examination. The records they used were extracted from the national registry system, which were reliable and continuous. And in both studies, the study sample was big and well screened to represent the ordinary being. So these data reflected the average coverage of the mammography screening and the relationship with reduced breast cancer mortality in a region of the country, which can be generalized to other territories and a larger population.
B. What are the applications or impacts of the findings? Mammography screening is cost-effective, feasible and affordable. So the population-based mammography screening programs should be recommended because it provides an operational framework and continuous improvement of the screening process and outcomes. And these programs identify and individually invite each person in the eligible population to attend each round of screening so that each person in the eligible population has an equal chance of benefiting from screening.
C. How can you apply the results to your practice? Since mammography screening has great efficiency in reducing breast cancer mortality. Being as a nurse practitioner, I should broadcast this program to my patients in clinical practice. I can offer health education that includes both general information about breast cancer risk and the importance of mammography screening. Culturally sensitive information can also be provided to reduce the knowledge barrier to screening and help patients understand their risk. After learning about their greater risk of breast cancer mortality, patients may have more motivation for scheduling regular mammograms. Besides, I will promote regular check up and ensure that my patients have an adequate screening round to obtain the maximum benefit of breast cancer prevention through mammography screening.
References
Domingo, L., Sala, M., Castells, X., Hofvind, S., Román, M., Hubbard, R., & Benkeser, D. (2015). Cross-national comparison of screening mammography accuracy measures in U.S., Norway, and Spain. European Radiology, 9p. doi:10.1007/s00330-015-4074-8
Otto, S. J., Fracheboud, J., Boer, R., De Koning, H. J., Verbeek, A. L. M., Otten, J. D. M., & ... Reijerink-Verheij, J. C. I. Y. (2012). Mammography screening and risk of breast cancer death: A population-based case - Control study. Cancer Epidemiology Biomarkers And Prevention, 21(1), 66-73. doi:10.1158/1055-9965.EPI-11-0476
Parkin, D. M., & Pisani, P. (1999). Global cancer statistics. CA: A Cancer Journal For Clinicians, 49(1), 33.
Pocobelli, G., & Weiss, N. S. (2015). Breast cancer mortality in relation to receipt of screening mammography: a case--control study in Saskatchewan, Canada. Cancer Causes & Control, (2), 231.