Regional trends in breast cancer incidence and mortality in Denmark prior to mammographic screening

Breast cancer mortality and overdiagnosis after implementation of population-based screening in Denmark

Introduction

Service breast cancer screening is difficult to evaluate because there is no unscreened control group. Due to a natural experiment, where 20% of women were offered screening in two regions up to 17 years before other women, Denmark is in a unique position. We utilized this opportunity to assess outcome of service screening.

Materials and methods

Screening was offered in Copenhagen from 1991 and Funen from 1993 to women aged 50–69 years. We used difference-in-differences methodology with a study group offered screening; a historical control group; a regional control group; and a regional–historical control group, comparing breast cancer mortality and incidence, including ductal carcinoma in situ, between study and historical control group adjusted for changes in other regions, and calculating ratios of rate ratios (RRR) with 95% confidence intervals (CI). Data came from Central Population Register; mammography screening databases; Cause of Death Register; and Danish Cancer Register.

Results

For breast cancer mortality, the study group accumulated 1,551,465 person-years and 911 deaths. Long-term breast cancer mortality in Copenhagen was 20% below expected in absence of screening; RRR 0.80 (95% CI 0.71–0.90), and in Funen 22% below; RRR 0.78 (95% CI 0.68–0.89). Combined, cumulative breast cancer incidence in women followed 8+ years post-screening was 2.3% above expected in absence of screening; RRR 1.023 (95% CI 0.97–1.08).

Discussion

Benefit-to-harm ratio of the two Danish screening programs was 2.6 saved breast cancer deaths per overdiagnosed case. Screening can affect only breast cancers diagnosed in screening age. Due to high breast cancer incidence after age 70, only one-third of breast cancer deaths after age 50 could potentially be affected by screening. Increasing upper age limit could be considered, but might affect benefit-to-harm ratio negatively.

As you like it: How the same data can support manifold views of overdiagnosis in breast cancer screening

Overdiagnosis estimates have varied substantially, causing confusion. The discussions have been complicated by the fact that population and study design have varied substantially between studies. To help assess the impact of study design choices on the estimates, we compared them on a single population. A cohort study from Funen County, Denmark, recently suggested little (∼1%) overdiagnosis. It followed previously screened women for up to 14 years after screening had ended. Using publically available data from Funen, we recreated the designs from five high-estimate, highly cited studies from various countries. Selected studies estimated overdiagnosis to be 25-54%. Their designs were adapted only to the extent that they reflect the start of screening in Funen in 1993. The reanalysis of the Funen data resulted in overdiagnosis estimates that were remarkably similar to those from the original high-estimate age-period studies, 21-55%. In additional analyses, undertaken to elucidate the effect of the individual components of the study designs, overdiagnosis estimates were more than halved after the most likely changes in the background risk were accounted for and decreased additionally when never-screened birth cohorts were excluded from the analysis. The same data give both low and high estimates of overdiagnosis, it all depends on the study design. This stresses the need for a careful scrutiny of the validity of the assumptions underpinning the estimates. Age-period analyses of breast cancer overdiagnosis suggesting very high frequencies of overdiagnosis rested on unmet assumptions. This study showed that overdiagnosis estimates should in the future be requested to adequately control for the background risk and include an informative selection of the studied population to achieve valid and comparable estimates of overdiagnosis.

Limitations in the Effect of Screening on Breast Cancer Mortality

Purpose

Randomized, controlled trials showed that screening reduces breast cancer mortality rates, but some recent observational studies have concluded that programmatic screening has had minor effect on breast cancer mortality rates. This apparent contradiction might be explained by the use of aggregated data in observational studies. We assessed the long-term effect of screening using individual-level data.

Materials and Methods

Using data from mammography screening in the Copenhagen and Danish national registers, we compared the observed breast cancer mortality rate in women invited to screening with the expected rate in absence of screening. The effect was examined using the “naïve model,” which included all breast cancer deaths; the “follow-up model,” which counted only breast cancer deaths in women diagnosed after their first invitation to screening; and the “evaluation model,” which is similar to the follow-up model during screening age, but after screening age, which counted only breast cancer deaths and person-years in women diagnosed during screening age.

Results

We included 18,781,292 person-years, 976,743 of which were from women invited to screening. The naïve and follow-up models showed, respectively, 10% and 11% reduction in breast cancer mortality after invitation to screening. However, many breast cancer deaths occurred in women whose cancer was diagnosed when they were no longer eligible for screening. Accounting for this dilution, the evaluation model showed a 20% (95% CI, 10% to 29%) reduction in breast cancer mortality after invitation to screening.

Conclusion

Screening had a clear long-term beneficial effect with a 20% reduction in breast cancer–associated mortality in the invited population. However, this effect was, by nature, restricted to breast cancer deaths in women who could potentially benefit from screening. Our study highlights the complexity in evaluating the long-term effect of breast cancer screening from observational data.

Benefit-to-harm ratio of the Danish breast cancer screening programme

The primary aim of breast cancer screening is to reduce breast cancer mortality, but screening also has negative side‐effects as overdiagnosis. To evaluate a screening programme, both benefits and harms should be considered. Published estimates of the benefit‐to‐harm ratio, the number of breast cancer deaths prevented divided by the number of overdiagnosed breast cancer cases, varied considerably. The objective of the study was to estimate the benefit‐to‐harm ratio of breast cancer screening in Denmark. The numbers of breast cancer deaths prevented and overdiagnosed cases [invasive and ductal carcinoma in situ (DCIS)] were estimated per 1,000 women aged 50–79, using national published estimates for breast cancer mortality and overdiagnosis, and national incidence and mortality rates. Estimations were made for both invited and screened women. Among 1,000 women invited to screening from age 50 to age 69 and followed until age 79, we estimated that 5.4 breast cancer deaths would be prevented and 2.1 cases overdiagnosed, under the observed scenario in Denmark of a breast cancer mortality reduction of 23.4% and 2.3% of the breast cancer cases being overdiagnosed. The estimated benefit‐to‐harm ratio was 2.6 for invited women and 2.5 for screened women. Hence, 2–3 women would be prevented from dying from breast cancer for every woman overdiagnosed with invasive breast cancer or DCIS. The difference between the previous published ratios and 2.6 for Denmark is probably more a reflection of the accuracy of the underlying estimates than of the actual screening programmes. Therefore, benefit‐to‐harm ratios should be used cautiously.

What's new?

Breast cancer screening reduces breast cancer mortality, but one negative side‐effect is overdiagnosis. Published estimates of the benefit‐to‐harm ratio–the number of prevented breast cancer deaths divided by the number of overdiagnosed breast cancer cases–vary considerably. This study reports a benefit‐to‐harm ratio of 2.6 for women invited to breast cancer screening in Denmark. Among 1,000 invited women from age 50 and followed up until 79, 2–3 women would be prevented from dying from breast cancer for every overdiagnosed woman. International variations in benefit‐to‐harm ratios probably reflect differences in the accuracy of underlying estimates more than differences between screening programmes.

Overdiagnosis in mammographic screening for breast cancer in Europe: a literature review

OBJECTIVES

Overdiagnosis, the detection through screening of a breast cancer that would never have been identified in the lifetime of the woman, is an adverse outcome of screening. We aimed to determine an estimate range for overdiagnosis of breast cancer in European mammographic service screening programmes.

METHODS

We conducted a literature review of observational studies that provided estimates of breast cancer overdiagnosis in European population-based mammographic screening programmes. Studies were classified according to the presence and the type of adjustment for breast cancer risk (data, model and covariates used), and for lead time (statistical adjustment or compensatory drop). We expressed estimates of overdiagnosis from each study as a percentage of the expected incidence in the absence of screening, even if the variability in the age range of the denominator could not be removed. Estimates including carcinoma in situ were considered when available.

RESULTS

There were 13 primary studies reporting 16 estimates of overdiagnosis in seven European countries (the Netherlands, Italy, Norway, Sweden, Denmark, UK and Spain). Unadjusted estimates ranged from 0% to 54%. Reported estimates adjusted for breast cancer risk and lead time were 2.8% in the Netherlands, 4.6% and 1.0% in Italy, 7.0% in Denmark and 10% and 3.3% in England and Wales.

CONCLUSIONS

The most plausible estimates of overdiagnosis range from 1% to 10%. Substantially higher estimates of overdiagnosis reported in the literature are due to the lack of adjustment for breast cancer risk and/or lead time.

Stage and survival in breast cancer patients in screened and non-screened Danish and Swedish populations

Comparisons between the Danish and Swedish Cancer Registry revealed a 9% difference in 5-year survival for breast cancer patients diagnosed between 1983 and 1989. The purpose of this study was to determine whether previous differences in survival and stage still exist or whether the Danish figures approach those of the Swedish registry. Complete population-based cohorts of patients with breast cancer in the years 1996-1997 from well-defined areas in Denmark and Sweden were compared. The study regions were a Danish (Funen) and a Swedish (Malmö) county with mammography screening and two Danish counties without screening. No difference in extent of disease or survival was observed between screening areas regardless of country. However, there were significant differences in stage distribution and survival between screening populations and Danish non-screening populations, to the benefit of populations provided with a screening programme. Five-year survival was 5-6%, higher in screening populations than in Danish non-screening counties. Corresponding disease-specific survival enhanced the difference. In a multivariate analysis increasing age, tumour size and stage decreased survival. Adjusting for these factors eliminated differences in survival according to country/county. Survival difference could be attributed to early diagnosis and favourable stage in populations offered mammography screening and was not related to nation. Denmark may, through early detection, approach the beneficial stage distribution and survival observed in Sweden.

Trends in breast cancer during three decades in Denmark: stage at diagnosis, surgical management and survival

BACKGROUND

Diagnostic and treatment of breast cancer has steadily improved in the last decades; mammography-screening, more aggressive axillary surgery and finally sentinel lymph node biopsies. This study analyses the impact of time trends in diagnostic and surgical procedures on stage at diagnosis and survival in three Danish counties in three decades.

METHODS

We compare extent of disease and outcome in three complete population-based patient cohorts with primary invasive breast cancer, diagnosed in 1986, 1996-1997 and 2002-2003; 3 385 patients, thereby comparing populations with and without centralised breast cancer management, mammography-screening, sentinel lymph node biopsies and the effect of time-period.

RESULTS

The 5-year over-all survival increased significantly (p <0.001) from 74% (70-78) in 1986 to 84% (82-86) in 2002-2003 with a corresponding increase in 10-year relative survival from 63% (58-68) in 1986 to 74% (71-77) in 1996-1997. In 1986 patients from Funen had median 10 lymph-nodes removed vs. median 5 and 7 in the other counties (p <0.001), and significantly more patients had positive lymph-nodes. In 1996-1997 the number of lymph-nodes removed increased significantly in all counties (median 12,13,14). In Funen, mammography-screening was implemented and patients had more favourable stage distribution (p <0.001). This was associated with significantly better over-all survival in univariate- but not in multivariate analysis. In 2002-2003 we found smaller tumors and more favourable stage-distribution in patients from Funen (p <0.001), and the 5-year relative survival increased to 90% (86-93). With increasing attention to the axilla more patients were found with positive nodal status, least in Funen (45% vs. 51% and 53%) (p <0.001). We found no significant differences in over-all survival according to county.

CONCLUSION

During these three decades over-all survival of breast cancer improved. In the earlier periods we found survival differences according to residence, but in the late cohort there were no difference despite better stage distribution in the county with mammography-screening.

Estimating the benefits of mammography screening: the impact of study design

BACKGROUND

Mammography screening is justifiable only if it leads to reduction in breast cancer mortality. However, evaluation of routine screening is not straightforward, as no unscreened control group is available. We report here on a cohort study of the effect of routine mammography on breast cancer mortality, and illustrate how variations in the analytic approach can affect the conclusions.

METHODS

We used data from the mammography screening program in Copenhagen, Denmark, for the period 1991-2001. We used local historical, concurrent regional, and historical regional control groups, and included only deaths from breast cancers diagnosed during the observation periods. We examined the impact of various control groups, of including all breast cancer deaths, and of using individual data versus routine statistics.

RESULTS

Combining all 3 control groups gave an estimated 25% reduction in breast cancer mortality. The estimate was 20% using only a local historical control group, and 9% using only a concurrent regional control group. Including all breast cancer deaths resulted in an estimate of 21% reduction in breast cancer mortality. Using routine statistics and a concurrent regional control group resulted in an estimated increase of 6% in breast cancer mortality.

CONCLUSION

Estimated changes in breast cancer mortality following the introduction of routine mammography ranged from a 25% reduction (based on the best methodology) to a 6% increase with a less rigid study design. The estimated effect of routine mammography on breast cancer mortality is thus highly dependent on study design.

Overdiagnosis, Sojourn Time, and Sensitivity in the Copenhagen Mammography Screening Program

The goal of this research was to estimate the overdiagnosis at the first and second screens of the mammography screening program in Copenhagen, Denmark. This study involves a mammography service screening program in Copenhagen, Denmark, with 35,123 women screened at least once. We fit multistate models to the screening data, including preclinical incidence of progressive cancers and nonprogressive (i.e., overdiagnosed) cancers. We estimated mean sojourn time as 2.7 years (95% confidence interval [CI] 2.2-3.1) and screening test sensitivity as 100% (95% CI 99.8-100). Overdiagnosis was estimated to be 7.8% (95% CI 0.3-26.5) at the first screen and 0.5% (95% CI 0.02-2.1) at the second screen. This corresponds to 4.8% of all cancers diagnosed among participants during the first two invitation rounds and following intervals. A modest overdiagnosis was estimated for the Copenhagen screening program, deriving almost exclusively from the first screen. The CIs were very broad, however, and estimates from larger datasets are warranted.

Breast cancer mortality in Copenhagen after introduction of mammography screening: cohort study

OBJECTIVES

To evaluate the effect on breast cancer mortality during the first 10 years of the mammography service screening programme that was introduced in Copenhagen in 1991.

DESIGN

Cohort study.

SETTING

The mammography service screening programme in Copenhagen, Denmark.

PARTICIPANTS

All women ever invited to mammography screening in the first 10 years of the programme. Historical, national, and historical national control groups were used.

MAIN OUTCOME MEASURES

The main outcome measure was breast cancer mortality. We compared breast cancer mortality in the study group with rates in the control groups, adjusting for age, time period, and region.

RESULTS

Breast cancer mortality in the screening period was reduced by 25% (relative risk 0.75, 95% confidence interval 0.63 to 0.89) compared with what we would expect in the absence of screening. For women actually participating in screening, breast cancer mortality was reduced by 37%.

CONCLUSIONS

In the Copenhagen programme, breast cancer mortality was reduced without severe negative side effects for the participants.

Time trends and regional differences in registration, stage distribution, surgical management and survival of breast cancer in Denmark

The aim of this study was to analyse time trends, stage at diagnosis, survival and registration of population-based cohorts of breast cancer patients in selected Danish counties (in total 2504) in 1986 and 1996-1997. In 1986, no differences in the extent of disease were observed between the counties. Patients from one county (Funen) had centralised surgery, significantly more lymph nodes removed and a better survival in the multivariate analysis. In 1996-1997, mammographical screening had been implemented in Funen, leading to a significantly better stage distribution, whereas stage remained unchanged in the other counties. In Funen, survival was significantly better than in the other counties in univariate, but not in multivariate analysis. Survival increased significantly with time only in Funen. Inclusion in clinical trials increased over time and the coverage of the database in the Danish Breast Cancer Cooperative Group (DBCG) was high. However, patients not notified in DBCG had, beside older age, also worse stage of disease distribution and less extensive surgery. A difference in survival was observed between the counties. In 1986, this may be explained by a centralised surgical system in one county, whereas in 1996-1997 improvements could be due to an early diagnosis and other as yet unknown factors. The DBCG database cannot be considered as representative of the Danish population of breast cancer patients.

Axillary surgery in breast cancer management--background, incidence and extent of nodal spread, extent of surgery and accurate axillary staging, surgical procedures

The prime objectives of axillary surgery in the management of breast cancer are 1) accurate staging, 2) treatment to cure and 3) quantitative information of metastatic lymph nodes for prognostic purposes and allocation to adjuvant protocols. It is generally agreed that axillary node status in potentially curable breast cancer is considered the single best predictor of outcome and the main determinant of allocation to adjuvant therapy. No physical examination, no imaging techniques, and no molecular biologic markers can today replace axillary surgery for staging purposes. The objectives of axillary surgery are best obtained by carrying out a complete axillary clearance. Nonetheless, less radical surgery is generally performed by carrying out a sampling procedure with a yield of about 4 nodes or a partial axillary dissection level I-II with at least 10 nodes recovered. Understaging the axilla is detrimental to outcome and, furthermore, locoregional tumor control is important for survival. Axillary surgery should therefore be conducted in accordance with high professional standards.

Breast cancer in Norway 1970-1993: a population-based study on incidence, mortality and survival

The incidence, mortality and survival of breast cancer patients from 1970 to 1993 were studied using data from the Cancer Registry of Norway. The age-adjusted incidence rate increased from 62.0 to 76.9 per 100,000 person-years during the period, and more than 2000 cases are now registered annually. The increase tends to be highest in the age group below 40 years. The increase is mainly found in cases with localized tumours at the time of diagnosis. The mortality rate has been almost unchanged in the period; the age adjusted mortality rate is 27.0 per 100,000 person-years at the end of the study period. The 5-year overall survival has increased among cases with axillary lymph node metastases at the time of diagnosis; the other stages show only little improvement.

Use of cancer incidence data in identification of cancer causation

This paper discusses the use of cancer incidence data in identification of cancer causation. Selective descriptive and analytical epidemiological studies were reviewed. These examples were taken primarily from Denmark, where the possibilities for epidemiological research are good due to the existence of many exposure and disease registers. Descriptive studies are still needed for a better understanding of cancer. Analytical studies of individual risk factors today often show relative risks of only 1.5 to 2, and these are difficult to translate into preventive recommendations. Epidemiology still remains the best available tool for identification of risk factors.