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Chapter 14: Impact of Mammography on U.S. Breast Cancer Mortality, 1975–2000: Are Intermediate Outcome Measures Informative?
Affiliations of authors: Department of Public Health, Erasmus MC University Medical Center Rotterdam, Rotterdam, The Netherlands (JDFH, SYGLT); Statistical Research and Application Branch, Division of Cancer Control and Population Science, National Cancer Institute, Bethesda, MD (KAC)
Correspondence to: J. Dik F. Habbema PhD, Department of Public Health, Erasmus MC University Medical Center Rotterdam, PO Box 2040, 3000 CA Rotterdam, The Netherlands (e-mail: j.d.f.habbema{at}erasmusmc.nl).
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ABSTRACT |
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Seven models have estimated the contribution of screening to the decrease in U.S. breast cancer mortality between 1975 and 2000. We will investigate whether the model estimates of the mortality reduction due to screening are associated with intermediate outcome measures (IOMs). Detection rates at screening, 1- and 2-year sensitivity, program sensitivity, and incidence of advanced tumors are used as IOMs. Moreover, the model parameters preclinical duration and sensitivity are analyzed. The correlation of IOMs with mortality is assessed for actual U.S. screening and for an intensive screening scenario, with annual screening at ages 40–79 years with 100% participation. Also, 12 alternative screening scenarios are run for one of the models, and within-model correlation between IOMs and mortality reduction is described. Resulting correlations between IOMs and mortality reduction are mostly weak. For 2-year sensitivity and the incidence of advanced tumors, correlations are high in the intensive screening scenario. Within-model correlations are strong for incidence of advanced tumors and program sensitivity. Intermediate outcome measures have limited potential in predicting the impact of mammographic screening on mortality. Incidence of advanced tumors and program sensitivity are measures that merit further consideration as surrogates for mortality reduction.
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INTRODUCTION |
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TopAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Between 1975 and 2000 there was a 24% decrease in age-adjusted breast cancer mortality in the United States for women between the ages of 20 and 79 years. This drop can at least partly be attributed to the introduction and dissemination of mammography screening for early detection of breast cancer and to the development and use of adjuvant chemotherapy and tamoxifen for treatment. Seven CISNET research groups have developed or adapted a model for explaining the U.S. breast cancer mortality trend 1975–2000. An overview of the background, organization, and work plan of the CISNET collaboration is found in (1). After 4 years of collaborative work, each model came up with a different estimate of the contribution of screening and adjuvant therapy in bringing down the U.S. mortality trend (2).
In cancer, mortality is an endpoint of an often long disease process, involving tumor initiation, growth, spread, diagnosis, and treatment. Especially in breast cancer, in which dissemination to distant organs can take place many years after diagnosis, mortality follow-up has to be long for assessing the final impact of screening. Predicting the magnitude of the mortality reduction from results in intermediate outcome measures (IOMs) would be helpful; such IOMs could be used as surrogate outcome measures. Good IOMs are relevant for screening, with its early cancer detection and the subsequent steps, eventually leading to a reduction in breast cancer mortality. For the impact of adjuvant therapy, an IOM analysis is not interesting because it involves only survival time, with diagnosis of distant metastases as the only possible IOM.
The association of IOMs with mortality will be assessed across the seven models. All models have been fitted to U.S. data. But they all have a different structure and different mechanisms linking natural history to mortality, and they also differ in how screening intervenes in the natural history (3–6).
We will consider the following observable IOMs: detection rate at first screen, detection rate at second and later screens, 1-year sensitivity, 2-year sensitivity, program sensitivity, and incidence of advanced tumors. To get more insight into the role of model assumptions, we will also study the model parameters sensitivity and duration of preclinical stage, which both can be assumed to be linked to mortality reduction by screening.
We will study both actual U.S. screening during 1975–2000 and an intensive screening scenario, which consists of annual screening of ages 40–79 with 100% participation during the same period. Associations between IOMs and mortality are likely to be stronger in the idealized situation of the intensive screening scenario, because there will be a larger mortality effect and one source of variability between models less, namely, how they deal with the screening dissemination and participation over time.
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MATERIALS AND METHODS |
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Seven breast cancer research groups and their corresponding models participate in the CISNET consortium (1). They will be abbreviated as follows: D, Dana-Farber ( 7); E, Erasmus MC (8); G, Georgetown (9); M, M. D. Anderson (10); R, Rochester (11); S, Stanford (12); W, Wisconsin (13).
The intermediate outcome measures are defined in Table 1. They are all empirical and can be calculated from the output of model simulation runs, irrespective of the model structure. The way of generating and processing output made calculating IOMs for some models prohibitive; see Table 2. For five of the models, both test sensitivity and preclinical duration could be calculated (D, E, G, S, W). Also, preclinical duration could be calculated for model R. Preclinical duration is the period that tumors can be detected by mammography screening but will not yet be diagnosed clinically. Test sensitivity is the probability of detecting a tumor through a mammography screening test, given that the tumor is in the preclinical period. See Tables 3 and 4 for how duration and sensitivity are embedded in the seven models. For other assumptions see (5,14–18).
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The two simulation runs that are analyzed for intermediate outcome measures are the actual screening scenario, reflecting the actual dissemination of screening in the United States (14), and an intensive screening scenario, see Table 5.
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The IOMs have been calculated for the whole period 1975–2000.The results have been age standardized for ages 30–79 to make results of the two scenarios comparable. For some IOMs, further simulations are undertaken. See "Results" for further explanation.
Throughout the paper the IOMs are correlated with mortality reduction attributable to screening. All models calculate this mortality reduction with respect to the mortality in 2000 in the hypothetical situation that no screening would have taken place.
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RESULTS |
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The values of the intermediate outcome measures and mortality reduction are given in Table 6. There is considerable variation between the models, roughly a factor of 3, between smallest and largest value (for sensitivity, the factor 3 applies to 1 minus sensitivity). The incidence rate of advanced tumors shows less variation for the actual than for the intensive screening scenario. There is enough variability between the models in the estimate of the percent mortality reduction for an interesting analysis of its association with IOMs. Unfortunately, it was possible for only one of the models (M) to calculate the uncertainty in the model outcome; see (10).
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Figure 1 gives the associations between IOMs and mortality reduction. In general correlations are low, or even in the unexpected direction. There are high correlations (>.75) for 2-year sensitivity and incidence of advanced tumors, both for the intensive screening scenario. For the actual screening scenario none of the measures gives a high correlation.
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Figure 2 gives a positive within-model association for the two sensitivity measures. The association is strong for the program sensitivity and less so for the 2-year sensitivity. The association for the detection rate and for the incidence rate of advanced tumors is, as to be expected, negative, and turns out to be strong for the incidence rate and less so for the detection rate. For the model parameter values for test sensitivity and preclinical duration, the association with mortality reduction was modest or absent, and their product [test sensitivity x preclinical duration], which reflects the potential of detecting a cancer at screening, was also showed not correlated with mortality reduction (data not shown).
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For one of the models (E), we studied the within-model association of the final mortality outcome with 2-year sensitivity, program sensitivity, and incidence rate of advanced tumors. We simulated 12 scenarios characterized by differences in screening interval and participation (Fig. 3). There is no association between the 2-year sensitivity and mortality reduction. For the program sensitivity and the incidence of advanced stages, on the other hand, the association is very strong with correlations of .99 and 1.00.
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Figure 3 has been calculated for data up to 2000, the year in which the mortality reduction by screening is assessed. To explore the predictive power of the IOMs, we assessed whether mortality reduction in 2000 could be predicted from early screening data. We therefore deleted screening data from the last 5, 10, and 20 years before 2000, using data up to 1995, 1990, and 1980. The correlation between program sensitivity and mortality reduction in the year 2000 remains .99. Comparable results were obtained for incidence of advanced tumors (data not shown).
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DISCUSSION |
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TopAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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The between-model association between intermediate outcome measures and mortality reduction was limited (Fig. 1). Moreover, the correlations were based on at most seven observations. The results of Fig. 1 must therefore be seen as primarily exploratory. We hope that they can inspire more research.
Although Figure 1 considers associations arising from differences between models, Fig. 2 explores for each of the models if the IOMs rightly reflect the higher effectiveness of the intensive screening scenario compared with the actual screening scenario. Although this relationship seems obvious, only program sensitivity and the incidence rate of advanced tumors show a consistent association; detection rate and 2-year sensitivity do not.
The scenario results in Figure 3 are better for the program sensitivity than for the 2-year sensitivity. This finding can be explained as follows. The 2-year sensitivity is closely related to the test sensitivity, because interval cases occur after a negative screening test. Its value also depends on the duration distribution of the preclinical screen detectable stage, because de novo fast-growing cancers can clinically surface shortly after a true negative mammography. The intensity of screening is not well reflected in this measure. Program sensitivity, on the other hand, does reflect the intensity of screening in the population because all cancers that are not screen detected are considered missed. Program sensitivity thus also reflects population coverage of the screening and the possible differential participation by high- and low-risk groups (the latter was in none of the models). The correlation of program sensitivity with mortality reduction in 2000 remains excellent (correlation = .99) when predicting the year 2000 mortality 20 years in advance. Here program sensitivity is based on screening results from 1975 to 1980 only. This preservation of prediction power looks encouraging, but is not surprising, because in the simulation, participation rate and test sensitivity were fully realized immediately in 1975. So in the scenarios, early screening results are fully representative for later results, which will not be the case with an actual screening program that develops and changes over time.
Table 6 presents the age-standardized values for IOMs. Underlying age-specific patterns can differ considerably between models. For example, when considering the duration of the preclinical screen detectable stage, in model D there is an increase in duration for ages 30–50 years and is constant at older ages. For models E and G there is a steady increase in duration from age 40 onward. For the other models the duration is about constant. These differences may lead to different recommendations on the screening age range for the models.
Standard screening theory would predict a clear positive association between sensitivity and mortality reduction because more cancers will be detected early when sensitivity is higher. Also there is a direct link between duration of the preclinical stage and mortality reduction because a longer preclinical stage means a longer possibility for early detection. However, we did not find strong correlations in our analysis. This outcome can to a large extent be explained by the multivariable character of the link between model parameters and mortality. For example, sensitivity and duration of the preclinical screen detectable stage can compensate for each other when explaining mortality results from screening. This effect is illustrated in Fig. 4. The ellipse in Fig. 4 represents a plausible area of relationships between values for sensitivity and duration in explaining mortality reduction from screening. The five crosses are examples of five hypothetical models that all give a reasonable fit to the observed data, but because of the different model structure they all come to a different plausible set of values. Mortality reduction is larger when moving to the right upper corner because in that direction both sensitivity and duration increase. The lower part of Fig. 4 shows the resulting relationship between duration and mortality reduction. The three points with the same mortality reduction have the smallest, middle, and largest value for duration. The extreme values for mortality reduction have intermediate values for duration. The resulting correlation is therefore modest. Because of the symmetry of the figure, the same results holds for the value of the sensitivity. In our situation, with duration and sensitivity having different meanings in each of the models (see Tables 3 and 4), the observed low correlations are therefore not surprising.
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To further illustrate the importance of a good fit of the models to screening data in explaining the lack of correlation between duration of preclinical stage and mortality reduction, we performed a small experiment: For model E we started with the duration assumption resulting from the fit to screening data (2.7 years), and we changed the duration to longer and shorter values without bothering about the fit to the data (which in fact dramatically worsened). As expected, we then obtained high correlations between duration and mortality reduction with values of .91 and .99 for the actual screening and the intensive screening scenario, respectively.
Throughout this paper, we sought to predict the percent mortality reduction by screening. However, in reality absolute mortality will be observed, and not a reduction with respect to the counterfactual situation of no screening. Since each model estimated the U.S. mortality in the absence of screening differently, the association of IOMs with mortality will not be the same as with mortality reduction. Moreover, developments over time in risk factors and treatment will influence mortality trends. Therefore, the IOM results here are not applicable for predicting mortality trends.
Thus far we considered correlations between IOMs and mortality reduction. Predicting mortality reduction should be based on a regression function that estimates the mortality reduction for each possible value of the IOMs. We visually inspected Figs. 2 and 3 in this respect. Program sensitivity looks promising because when extrapolating the lines to the left all lines but one in Fig. 2 and the curve in Fig. 3 go through the origin, reflecting that zero program sensitivity will lead to zero mortality reduction. For the incidence rate of advanced tumors we also have a similar result because when extrapolating the lines to the right, four of the lines predict that an incidence rate of advanced tumors of about 15–18 per 100 000 will lead to no mortality reduction for the screening program. This is indeed roughly the incidence of advanced tumors in the absence of screening. Interestingly, this incidence of 15–18 is also obtained with extrapolation of a regression line through the between-model observations in Fig. 1 (intensive screening scenario). A meaningful result for a prediction regression line was obtained for none of the other measures.
A comparison of intermediate outcome measures between the actual screening scenario and the intensive screening scenario offers an opportunity for assessing the validity of the models. There are a few unexpected results (see also Fig. 2). One of the models (M) gives an only slightly greater mortality reduction for the intensive screening scenario. The detection rate in second and later screens did not change between the two scenarios for model W, although one would expect lower detection rates with more intensive screening. The detection rate at first screening in the intensive screening scenario is relatively low for W and high for M. And the 2-year sensitivity for R was lower for the intensive screening scenario, although in this scenario interval cancers can occur only in the first year after a negative screening because of the yearly screening with 100% participation. The occurrence of several counterintuitive or outlying values for a model does not necessarily imply lack of validity; these values may concern compensating mechanisms within the complex model, together leading to a good fit of the reference data. Also, alternative pathways from screening to mortality reduction are possible. For example, the incidence rate for advanced tumors in model M is not changed by screening, because this model uses a combination of stage shift for earlier stage disease only and survival benefit for all screen-detected cases. Thus a stage shift for stage III and IV will not occur, and the incidence of advanced tumors will therefore also not change between actual screening and intensive screening in Fig. 2. The occurrence of counterintuitive results warns us that model structures that are appropriate for addressing the base case question may not always be appropriate for addressing other problems in which the answer depends heavily on these results.
The relationship between IOMs and mortality reduction was also assessed for the two model parameters sensitivity and preclinical duration. The weak and inconsistent associations might have been caused by the different definitions of these parameters in the seven models (see Tables 3 and 4). Especially the preclinical sojourn time has a more straightforward meaning in simple progression models than more complex natural history models, which include tumors (mostly in the in situ and early invasive stages) that regress, do not progress, or may even progress and regress. This difference in meaning reminds us that comparison of parameter values across different models should never be done naively; it should take the exact role of the parameter in each model into account.
The quest for surrogate outcome measures is an old one. The methodology has been developed in (19,20). Although the discussion of IOM in these papers was mainly in the context of clinical trials, the reasons for the conclusion that intermediate endpoints are not easily good surrogates of the final outcome (21) have a more general validity. It is important to use IOMs that a priori are least likely to be biased with respect to the final outcome. The number of advanced cancers is an attractive measure, because they can be detected in an earlier stage by screening. And because they often lead to mortality, their number is closely related to the final outcome measure of mortality. However, when the effect of screening on mortality is primarily induced by so-called within stage shift, the number of advanced cancers may be less important as an IOM. Time trends and incidence will bias this measure, but it will bias the final mortality outcome in the same direction. Program sensitivity is an attractive surrogate outcome measure, because only the fraction of cancers that are early detected can contribute to mortality reduction. This measure will be biased by a relatively high detection rate of favorable-prognosis cancers, including nonprogressive ones.
Stage distribution is a dangerous IOM because ineffective screening can nevertheless lead to a more favorable stage distribution by detection of indolent or regressive cancers. For example, the stage distribution of lung cancer in the Mayo clinics trial (22) is cited by (21) as an example of a failure of surrogate endpoints for screening. When, however, for the same trial the early lung cancer stages would not have been taken into account and only advanced stages would have been counted for calculating the IOM "incidence of advanced tumors," nearly no difference between screening and control group would have been found, which is an early reflection of the later no-effect conclusion of the Mayo trial, based on mortality follow up.
Complexity of the models, interplay of the model parameters, and inconsistently defined terms make recognizing and interpreting relationships between the IOMs and mortality difficult. Although IOMs are informative and contribute to the understanding of the individual models, they are not necessarily good predictors of mortality reduction due to screening. Two IOMs that merit further consideration as surrogate outcome measures because of their performance and their plausible link to screening effectiveness are incidence of advanced tumors and program sensitivity.
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