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1 December 1997 | Volume 127 Issue 11 | Pages 955-965
Background: Screening mammography is recommended for women 50 to 69 years of age because of its proven efficacy and reasonable cost-effectiveness. Extending screening recommendations to include women 40 to 49 years of age remains controversial.
Objective: To compare the cost-effectiveness of screening mammography in women of different age groups.
Design: Cost-effectiveness analysis done using Markov and Monte Carlo models.
Patients: General population of women 40 years of age and older.
Interventions: Biennial screening from 50 to 69 years of age was compared with no screening. Screening done every 18 months from ages 40 to 49 years, followed by biennial screening from ages 50 to 69 years, was compared with biennial screening from ages 50 to 69 years.
Measurements: Life-expectancy, costs, and incremental cost-effectiveness.
Results: Screening women from 50 to 69 years of age improved life expectancy by 12 days at a cost of $704 per woman, resulting in a cost-effectiveness ratio of $21 400 per year of life saved. Extending screening to include women 40 to 49 years of age improved life expectancy by 2.5 days at a cost of $676 per woman. The incremental cost-effectiveness of screening women 40 to 49 years of age was $105 000 per year of life saved. On the basis of a multiway sensitivity analysis, there is a 75% chance that screening mammography in women 50 to 69 years of age costs less than $50 000 per year of life saved, compared with a 7% chance in women 40 to 49 years of age.
Conclusion: The cost-effectiveness of screening mammography in women 40 to 49 years of age is almost five times that in older women. When breast cancer screening policies are being set, the incremental cost-effectiveness of extending mammographic screening to younger women should be considered.
Two recently published analyses [20, 21] suggest that mammographic screening in younger women may be as cost-effective as screening in older women. The first analysis [20] calculated average cost-effectiveness by comparing a strategy of screening 40- to 69-year-old women with no screening. Most of the benefit achieved by using this strategy occurs when women are 50 to 69 years of age. Therefore, this analysis did not address whether it is cost-effective to screen women from 40 to 49 years of age in addition to screening them from 50 to 69 years of age. To determine whether the additional benefit obtained by extending screening mammography to women 40 to 49 years of age comes at a reasonable cost would require an incremental cost-effectiveness analysis [22-26]. The second analysis [21] used a simplified life-expectancy accounting method, did not discount costs or benefits, and associated screening mammography with unsubstantiated mortality reductions (30% for the base case). Neither analysis [20, 21] included an important aspect of the results of screening mammography trials in 40- to 49-year-old women: that is, no benefit occurs until 10 years after the initiation of screening.
An earlier analysis [10] found screening mammography to be more expensive in women 40 to 49 years of age than in women 50 years of age and older. This previous analysis calculated incremental cost-effectiveness, discounted costs and benefits, and included an estimated delay between the onset of screening and the onset of a mortality benefit. Our analysis extends this work by including updated pooled results of the randomized, controlled trials [19]; actual delay times before the onset of benefits; and updated costs of mammography and treatment of breast cancer.
We developed a Markov model [27, 28] that compared the life expectancy of women undergoing different breast cancer screening strategies. Except for women in whom breast cancer was diagnosed at the initiation of screening, women were healthy at entry into the model. At the end of each 1-year cycle, women were in one of four health states: They remained healthy, developed breast cancer and remained alive, died of breast cancer, or died of another cause. The transition probabilities [that is, the probabilities of developing breast cancer, dying of breast cancer, and dying of another cause] were both age- and strategy-dependent. The base-case analysis compared three strategies: 1) no screening; 2) screening biennially from 50 to 69 years of age; and 3) screening every 18 months from 40 to 49 years of age, followed by screening biennially from 50 to 69 years of age. The rationale for these screening intervals is discussed below. We calculated the cost-effectiveness of screening in women 50 years of age and older by comparing the first strategy with the second strategy. To determine the incremental cost-effectiveness of screening in 40- to 49-year-old women, we compared the second and third strategies. Costs and benefits were discounted at a rate of 3% per year for the base-case analysis [23].
Benefits
Trials of screening mammography have shown no reduction in breast cancer mortality among screened women until several years after the initiation of screening [5, 18, 29, 30]. Meta-analyses and one pooled analysis have shown that among 40- to 49-year-old women, the summary relative risk reduction in breast cancer mortality 7 to 9 years after the initiation of screening is about 1, indicating no reduction in mortality [5, 16-18]. Ten to 12 years after the initiation of screening, a nonsignificant trend toward reduced mortality is evident in the screened group (Figure 1, left) [5, 18, 29, 30]. Recently updated results show a statistically significant 16% reduction that occurs 10 to 14 years after the initiation of screening [19]. For women 50 to 69 years of age, there is an initial period of about 5 years that shows no benefit from screening (Figure 1, right) [18, 29, 30]. ARTICLE
Cost-Effectiveness of Extending Screening Mammography Guidelines To Include Women 40 to 49 Years of Age
There is universal agreement [1-4] that women 50 to 69 years of age should undergo screening mammography because randomized, controlled trials have shown that such screening reduces breast cancer mortality in this age group [5, 6]. This consensus is bolstered by the results of cost-effectiveness analyses that consistently show that this benefit can be achieved at a reasonable cost [7-9]. In contrast, whether women 40 to 49 years of age should undergo screening mammography is controversial [10-15]. Pooled results of large randomized, controlled trials have shown no mortality reduction in 40- to 49-year-old women after 7 to 9 years of screening [5, 16-18]. However, a statistically significant reduction in breast cancer mortality becomes apparent 10 to 14 years after the initiation of screening [19]. Some authors have argued that this delayed benefit should not be ignored [11]. However, the reality of constrained health care resources requires that any benefit from preventive services be achieved at a reasonable cost.
Methods
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Methods
Results
Discussion
Author & Article Info
References
Model
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In our model, for women who start screening at 50 years of age, a 27% reduction in breast cancer mortality (Table 1) [5] begins 5 years after the initiation of screening and continues until age 74 years. Although screening ends at 69 years of age, we assumed that women would continue to benefit for another 5 years because of early detection of breast cancer in the last years of screening. For women who begin screening at 40 years of age, a 16% reduction in breast cancer mortality starts at age 50 years; this reduction increases to 27% at age 55 years.
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Screening Interval
The screening interval in randomized, controlled trials of screening mammography has varied from 12 to 33 months for women 50 years of age and older. Pooled results of the efficacy of mammography stratified by length of screening interval do not differ for women in this age group [5]. From published results [5], we determined a 28% (95% CI, 15% to 31%) reduction in breast cancer mortality in women 50 years of age and older who were screened every 18 to 33 months and a 25% (CI, 1% to 43%) reduction in those screened every 12 months. For the base-case analysis, we therefore chose to perform biennial screening in women 50 years of age and older because screening more often only increases cost without increasing the benefits of screening. For the base-case analysis, we used the pooled reduction in breast cancer mortality (27%) [5] from all randomized, controlled trials to determine the cost-effectiveness of biennial screening in 50- to 69-year-old women; in a sensitivity analysis, we determined the cost-effectiveness of annual screening.
The screening interval in randomized, controlled trials has varied from 12 to 24 months for women 40 to 49 years of age. Pooled results of the efficacy of screening mammography stratified by length of screening interval did not show a statistically significant reduction in breast cancer mortality for 12-month or 18- to 24-month screening intervals [5]. As noted above, recently reported pooled results of all randomized, controlled trials, which on average used a screening interval of 18 months, showed a statistically significant 16% reduction in breast cancer mortality 10 to 14 years after the initiation of screening [19]. We therefore assumed that a 16% reduction in breast cancer mortality would be achieved with screening done every 18 months. This screening interval is consistent with the guidelines of organizations [2, 3] that recommend screening every 1 to 2 years for 40- to 49-year-old women. In sensitivity analyses, we calculated the cost-effectiveness of annual and biennial screening, assuming the same 16% reduction in breast cancer mortality among screened women.
Utilities
Because there are few data on the utility that women place on life after treatment of breast cancer or the utility placed on living with metastatic breast cancer, we did not include utilities in the base-case analysis. Data from a small Australian study [33] (which observed a utility of about 0.8 for life after treatment of breast cancer and a utility of about 0.3 for life with metastatic cancer) are included in a sensitivity analysis to determine the extent to which cost per year of life saved might differ from cost per quality-adjusted life-year saved.
Costs
We included three costs: the cost of screening mammographic examinations, the cost of evaluating abnormal mammograms, and the cost of treating breast cancer (Table 2). Additional details on derivations of costs are given in the Appendix. The cost of screening mammography was based on the average cost ($91) reported by the National Cancer Institute's National Survey of Mammography Facilities [34]. This cost was inflated to 1995 dollars ($106) by using the consumer price index for medical services. We assumed that women in whom breast cancer was diagnosed continued to undergo screening mammography of the opposite breast, at the same cost, after the initial diagnosis of breast cancer.
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The cost of evaluating abnormal mammographic results was calculated as a weighted average of procedures that may follow abnormal mammograms. This cost was also inflated to 1995 dollars. The distribution and types of follow-up procedures were based on those reported by the National Cancer Institute's National Survey of Mammography Facilities [35]. A range of costs for each procedure was based on data from Medicare, Pennsylvania Blue Cross, Group Health Cooperative, and Kaiser Permanente (Brown M. Personal communication). The percentage of abnormal mammograms was based on the percentage seen with high-quality modern screening mammography (Table 2) [36].
Population-based data on the cancer stage at diagnosis in screened compared with nonscreened women are sparse. Only one population-based study has reported the distribution of cancer stages according to screening status [37]. Using these results, we assumed that among 40- to 49-year-old women, there would be a 10% shift from stage II or higher cancer to stage I cancer (rate of stage I cancer increased from 51% to 56%) and a 40% shift in 50-to 69-year-old women (rate of stage I cancer increased from 49% to 69%). To determine the distribution of cancer stage among screened women, we applied these percentage shifts in cancer stage to the distributions of cancer stage reported by Surveillance Epidemiology and End Results (SEER) in 1982 (before the widespread use of screening mammography). The distribution of cancer stages in the nonscreened group was based on those reported by SEER in 1982. Data [38] used to calculate treatment costs (Table 2) in screened and nonscreened groups are given in the Appendix.
Other Probabilities and Assumptions
The overall mortality rate was estimated by using life tables (Table 1) [31]. The incidence of breast cancer was based on 1992 data from SEER [32]. On the basis of 1982 data [32], the mortality rates among nonscreened women with breast cancer were 33% at 10 years for 40- to 49-year-old women and 35% at 10 years for 50- to 69-year-old women.
Sensitivity Analysis
We performed three types of sensitivity analyses. The potential impact of each assumption (Table 1 and Table 2) was determined with a one-way sensitivity analysis that systematically varied the assumption throughout its range of values.
The second type of one-way sensitivity analysis was done by Monte Carlo simulation. This analysis examined the uncertainty surrounding the mortality reductions associated with screening. This Monte Carlo model was identical to the base-case Markov model except that the reductions in breast cancer mortality due to screening were entered as probability distributions based on the 95% CIs reported in pooled analyses [5, 19]. A new value from within the probability distribution of the mortality reduction was randomly selected during each of 1000 iterations; this resulted in a probability distribution of cost-effectiveness ratios. On the basis of this distribution, we determined how likely certain levels of cost-effectiveness were within the context of our model.
Finally, we performed a multiway Monte Carlo simulation to determine how likely certain levels of cost-effectiveness were when we simultaneously incorporated all ranges of values for variables listed in Table 1 and Table 2. We again performed 1000 iterations; in each iteration, we used newly selected values from within the ranges. Details of the Monte Carlo methods are available from the authors upon request.
Results
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Compared with no mammographic screening, a biennial screening mammography program for 10 000 women 50 to 69 years of age resulted in an additional 329 years of life (Table 3). The increase in life expectancy per woman was 12 days. By extending the mammography program to include every-18-month screening for 40- to 49-year-old women, an additional 64 years of life were saved. This extended each woman's life expectancy by an additional 2.5 days. If the benefits of mammographic screening were not discounted, life expectancy was improved by 43 days for women who began screening at 50 years of age and improved an additional 6.5 days if screening began at 40 years of age. Incorporating utilities into the model resulted in quality-adjusted life-years saved that were similar to the unadjusted life-years saved (Table 3).
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The cost of screening mammography from 50 to 69 years of age (compared with no screening) was $704 per woman (Table 3). The incremental cost of screening women from ages 40 to 49 years (screening from ages 40 to 69 years compared with screening from ages 50 to 69 years) was $676 per woman. Although treatment costs accounted for most of the costs in each strategy, the differences between the strategies (incremental costs) depended primarily on the different costs of screening.
On the basis of these results, we found the cost-effectiveness of screening 50- to 69-year-old women to be $21 400 per year of life saved and the incremental cost-effectiveness of screening 40- to 49-year-old women to be $105 000 per year of life saved. Results for other screening intervals (assuming no change in efficacy) are also reported in Table 3. Discounting costs and benefits at a rate of 5% resulted in higher cost-effectiveness ratios (compared with discounting at a rate of 3%) of $39 300 for 50- to 69-year-old women and $172 500 for 40- to 49-year-old women.
We also disaggregated the benefits of screening mammography (Table 4). If 10 000 40-year-old women did not undergo screening mammography at all, 308 would die of breast cancer by 80 years of age (Table 4). A total of 3546 would die of other causes. A biennial screening mammography program applied to this cohort from 50 to 69 years of age would avert 37 deaths; 52 deaths from breast cancer would be prevented, but 15 of these women would die of other causes by 80 years of age. Expanding the screening program to include every-18-month screening for 40- to 49-year-old women would avert an additional 4 deaths (for a total of 41); 6 deaths from breast cancer would be prevented, but 2 of these women would die of another cause by 80 years of age.
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Sensitivity Analyses
The results of the one-way Markov sensitivity analyses for 11 variables that may affect the incremental cost-effectiveness of screening 40- to 49-year-old women are shown in Figure 2. Two variables specific to screening 50- to 69-year-old women were not included. Varying the discount rate had the largest impact on the results. The mortality reduction associated with screening mammography in younger women was varied from 1 SD below to 1 SD above the point estimate. A mortality reduction of 9% resulted in a cost-effectiveness ratio of $186 500; a 23% mortality reduction resulted in a cost-effectiveness ratio of $73 000. The length of the delay between the onset of screening and the onset of a mortality reduction had a large impact on the results. In fact, if the true delay until the onset of a mortality reduction is 12 years (the midpoint between 10 and 14 years [19]), then the cost-effectiveness ratio is nearly $200 000 per year of life saved. The cost of screening mammography also had a great effect on the results. For the incremental cost-effectiveness ratio to decrease to less than $50 000 per year of life saved, the cost of screening mammography would have to be $45 or less. Finally, varying the screening interval from annual to biennial screening also had a large impact on the results, assuming that biennial screening is as effective as annual screening.
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Varying the percentage of abnormal mammograms had a moderate impact on the results. The cost to work up a patient with an abnormal mammographic result had a minimal impact on the results when a low percentage of mammograms (2% to 3%) were assumed to be abnormal. However, when the percentage of abnormal mammograms was set at the upper bound (11%), the cost to work up a patient with an abnormal mammographic result had a greater impact. Varying the utility placed on life after treatment for breast cancer and on life with metastatic breast cancer each had a moderate impact on the results. In the base-case analysis, the stage shift for 40- to 49-year-old women in the screened group was 10%. Varying this difference from 0% to 40% had almost no effect on the results.
Monte Carlo Analyses
As described in the Methods section, we performed a Monte Carlo simulation by using the CIs reported for the summary relative risk reductions in breast cancer mortality due to screening mammography [5, 19]. Of 1000 iterations, 99.5% resulted in cost-effectiveness ratios less than $50 000 per year of life saved for 50- to 69-year-old women. The 25th, 50th, and 75th percentiles for these women were $18 900, $21 400, and $24 000 per year of life saved, respectively. In sharp contrast, 0.2% of the 1000 iterations resulted in a cost-effectiveness ratio less than $50 000 per year of life saved for 40- to 49-year-old women. The 25th, 50th, and 75th percentiles for these women were $83 100, $105 000, and $146 400 per year of life saved, respectively. According to this analysis, 95% of values for 50- to 69-year-old women fell between $16 200 and $31 900 per year of life saved, whereas 95% of values for 40- to 49-year-old women fell between $63 500 and $340 200 per year of life saved.
We then performed a multiway sensitivity analysis using a Monte Carlo model that allowed all assumptions in the model to be varied simultaneously. According to this analysis, there was a greater than 75% chance that screening mammography for 50- to 69-year-old women would cost less than $50 000 per year of life saved; there was a less than 7% chance that this level of cost-effectiveness would be achieved by screening 40- to 49-year-old women. The 25th, 50th, and 75th percentiles for women 50 to 69 years of age were $17 300, $29 700, and $49 800 per year of life saved, respectively. The respective percentiles for women 40 to 49 years of age were $101 000, $167 200, and $299 100 per year of life saved. According to this analysis, 95% of values for women 50 to 69 years of age fell between $7200 and $109 100 per year of life saved, whereas 95% of values for women 40 to 49 years of age fell between $43 000 and $1 624 400 per year of life saved.
When we compared the results of the multiway Monte Carlo analysis with the results of the one-way Monte Carlo analysis, it was clear that our base-case model favors screening because the 50th percentile values for the one-way analysis are lower than the 50th percentile values for the multiway analysis. This can also be seen in Table 1 and Table 2. Several of the base-case values lie on one end of the range of plausible values. In each case, values favoring screening were chosen for the base-case model. Using the base-case model, we found that for screening mammography in younger women to be cost-effective at a level of less than $50 000 per year of life saved, the delayed reduction in breast cancer-related mortality that begins to occur at 50 years of age would have to be 34%. We also determined the extent to which this risk reduction is consistent with pooled results from randomized, controlled trials [5, 19] and, therefore, the extent to which the reduction is likely to be true. A risk reduction of 34% is 2.75 SDs from the point estimate of 16%; thus, there is a less than 0.5% chance that the true reduction is as great or greater (Table 5).
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Finally, we used the base-case model to calculate the number of 40-year-old women who would need to be screened to prevent one death by 80 years of age. If we assume that the true reduction in mortality due to screening is 16%, the number of women needed to be screened (every 18 months for 10 years) to prevent one death is 2500. If we assume that the true reduction in mortality for women 50 to 69 years of age is 27%, the number needed to screen (biennially for 20 years) to prevent one death is 270.
Discussion
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Our base-case analysis favored screening in several ways. First, we assumed that all of the benefit observed 10 to 14 years after the initiation of screening is attributable to screening that takes place while women are still between 40 and 49 years of age. By using data from the screening mammography trials, de Koning and Boer [39] recently showed that about 30% to 40% of the benefit observed 10 to 14 years after the initiation of screening in women 40 to 49 years of age is attributable to screening that occurs after women are 50 years of age or older. Had we included this in our model, the incremental cost-effectiveness ratio of screening mammography for 40- to 49-year-old women would have increased from $105 000 to $161 400 per year of life saved. On the cost side, we assumed that only 1.2 procedures would be performed per abnormal mammographic result, although others [40] have reported an average of two procedures per abnormal mammogram. We also based the percentage of abnormal mammographic results on those that would occur with a high-quality university screening program. Nationwide, the percentage of abnormal mammograms may be much higher [35]. Despite these assumptions favoring screening in younger women, mammography cost more than $50 000 per year of life saved in nearly all iterations of the one-way Monte Carlo analysis. Other researchers have noted that cost-effectiveness thresholds are controversial [41]. Nevertheless, programs that cost less than $50 000 per year of life saved (or per quality-adjusted life-year saved) are generally viewed favorably. Even when we varied all of our assumptions in a multiway sensitivity analysis, the likelihood that screening mammography costs less than $50 000 per year of life saved among 40- to 49-year-old women was only 7%; this finding demonstrates the robustness of our conclusion that screening mammography is costly in younger women.
Given the differences in the cost-effectiveness of screening mammography that we observed between younger and older women and the robustness of our results, how did previous analyses conclude that screening in younger women could be nearly as cost-effective as screening in older women? One analysis [20] calculated average cost-effectiveness When this measure is calculated, the cost-effectiveness ratios of programs that start screening at 40 years of age are low because the large benefit accrued from continued screening when women are older is averaged with the lesser benefit gained by screening younger women. In our model, for example, the average cost-effectiveness of screening 40- to 69-year-old women is $35 100. Incremental cost-effectiveness analysis, as presented here, counts only those costs and benefits specific to screening younger women. Another analysis [21] used unsubstantiated mortality reductions. Finally, both of these analyses assumed that screening leads to a mortality reduction starting in the first year, an assumption unsupported by the results of any prospective trial.
Although our model was somewhat sensitive to the length of screening interval, this sensitivity reflects the different costs associated with more frequent screening, not changes in efficacy. For women 50 years of age and older, biennial screening is clearly as efficacious as annual screening [5]. Therefore, the additional costs of annual screening are hard to justify. For women younger than 50 years of age, published data are insufficient to draw an evidence-based conclusion about the efficacy of mammography according to various screening intervals. We assumed that efficacy would not change with a longer or shorter screening interval. However, if a shorter screening interval (for example, annual or 18-month screening) is required to maximize efficacy, then strategies that use a longer screening interval (for example, biennial) may be less cost-effective than strategies that use a shorter screening interval.
Our analysis identified several factors that minimally affect cost-effectiveness ratios for screening mammography. As long as women have access to high-quality mammography with a relatively low rate of abnormal mammograms, the cost to work up an abnormal result has little effect on the cost-effectiveness ratios. If a large percentage of women attend screening programs that have a high rate of abnormal mammographic results (
11%) (and if most of these women do not have cancer) [35], then the effect of the cost to work up an abnormal result will be greater and screening mammography will be even less cost-effective. The utility that women place on living with breast cancer also has minimal impact on the results. Nevertheless, further research might change this conclusion if the utility that women with terminal breast cancer place on life is low. Finally, we found that although screening leads to lower costs of breast cancer treatment, these savings are small. Furthermore, because treatment costs apply only to the minority of women who develop breast cancer, they have almost no impact on the cost-effectiveness of screening mammography programs applied to a general population (most of whom will not develop breast cancer).
Our study has several limitations. We did not consider the benefit of reassurance associated with a normal mammographic result. We also did not consider that some screened women may benefit psychologically from less disfiguring procedures available to treat earlier-stage disease. We did not include indirect costs, such as productivity lost as a result of premature death from breast cancer. Had we included these factors, the cost-effectiveness ratios would have decreased. On the other hand, we did not consider the harm caused by false-positive test results or productivity lost as a result of screening and follow-up procedures. Finally, we did not consider the additional medical costs (for example, costs of treating heart disease) incurred by women saved by screening mammography. Had we included these factors, the cost-effectiveness ratios would have increased slightly.
How should society respond to the knowledge that screening younger women may be costly, whereas screening older women is reasonable from a cost-effectiveness standpoint? We offer three possible responses. First, society may choose to reject cost-effectiveness data with regard to this question. Given that younger women worry more about breast cancer, overestimate their risk for dying of breast cancer, and overestimate the benefit of mammography screening [42, 43], society may be unwilling to exclude young women from screening programs on the basis of cost-effectiveness. A second option would be to somehow decrease the cost-effectiveness of screening. As mentioned above, if mammography were much cheaper (
$45), reasonable cost-effectiveness could be achieved. Given that health practitioners are offering routine mammography screening to 40- to 49-year-old women, decreasing the cost of mammography screening should be an important national goal. Finally, society might decide that screening mammography in younger women is too costly and that health care resources should be directed toward interventions that have been proven to save lives at a reasonable cost.
In conclusion, screening mammography is relatively cost-ineffective among women 40 to 49 years of age because mammography is less efficacious and the incidence of breast cancer is low in this age group. To prevent one death among women 40 to 49 years of age, clinicians would need to routinely screen 2500 women. The likelihood that this screening would cost less than $50 000 per year of life saved is less than 7%. On the other hand, routinely screening only 270 women from ages 50 to 69 years also prevents one death. The likelihood that this screening costs less than $50 000 per year of life saved is greater than 75%. Given these results, mammography screening programs should focus on 50- to 69-year-old women, in whom the benefits are clear and the costs are acceptable. Extending screening to 40- to 49-year-old women could be costly to society. The incremental cost-effectiveness of extending mammographic screening to younger women should be considered when policies for breast cancer screening are being set.
Appendix
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Costs of breast cancer treatment were based on the costs reported for patients in Group Health Cooperative [38], a Washington State staff-model health maintenance organization with 388 000 members. Total cost to treat breast cancer consisted of the costs of initial care, continuing care, and terminal care. Taplin and colleagues [38] present cost data for carcinoma in situ and local, regional, and distant disease at presentation. Because cost data for distant disease were incomplete, we used cost data for local and regional disease. Costs of local disease were used for stage 1 disease, and costs of regional disease were used for stage 2 and higher disease. If the total cost to treat distant disease is greater than the cost to treat regional disease, we may have underestimated treatment costs. However, women who present with distant disease make up a small proportion of all patients with breast cancer. The effect on treatment cost savings in our model would therefore be small. Our model did not consider carcinoma in situ because the impact of detecting such carcinoma on reducing breast cancer mortality is unknown. Therefore, we did not include the cost to treat carcinoma in situ.
Taplin and colleagues [38] separately reported results stratified by age and by disease stage. Data stratified simultaneously by age and stage were not presented. We therefore estimated age-specific cost data according to stage in the following manner. The average total cost to treat women younger than 50 years of age was $41 065. We divided this cost into the cost of treating the 56% of screened women younger than 50 years of age who had local disease and the 44% of screened women in this age group who had regional disease. In general, local disease costs 79% as much as regional disease ($26 755 compared with $34 019 for all ages). Using this information, we solved for the cost of local and regional disease in screened younger women ($36 683 and $46 642, respectively). The average total cost to treat women 50 years of age and older was $33 576. We divided this cost into the cost of treating the 69% of screened women 50 years of age and older who had local disease and the 31% of screened women in this age group who had regional disease. Using this information, we solved for the cost of local and regional disease in screened older women ($30 969 and $39 378, respectively).
Women who do not undergo screening tend to present with higher-stage disease. For women younger than 50 years of age, there is a 10% stage shift (with screening, 10% of women in the group with stage 2 or higher disease move to the group with stage 1 disease); for women 50 years of age and older, the shift in disease stage is 40%. On the basis of these data, 51% of nonscreened women younger than 50 years of age present with stage 1 disease. Therefore, the total cost of treating nonscreened women younger than 50 years of age is $41 563 (0.51 x 36 683 + 0.49 x 46 642). Similarly, the total cost of treating nonscreened women 50 years of age and older is $35 258 (0.49 x 30 969 + 0.51 x 39 378). In other words, treatment of nonscreened women is 1.2% more expensive for women younger than 50 years of age ($41 563 compared with $41 065) and 5% more expensive for women 50 years of age and older ($35 258 compared with $33 576). These small differences are further diminished by the fact that they apply only to women who develop breast cancer. Therefore, the saving in treatment costs per woman screened (most of whom will never develop breast cancer) is small, and errors in our estimation of treatment cost would have to be large to influence the cost-effectiveness ratios.
The types of follow-up procedures were based on the National Survey of Mammography Facilities [34], and the costs of these procedures were based on data from Medicare, Pennsylvania Blue Cross, Group Health Cooperative, and Kaiser Permanente (Brown M. Personal communication). An average of 1.2 procedures were done per abnormal mammographic result, and 11% of mammograms were abnormal; 1000 screening mammograms resulted in 47.1 special views, 40.2 repeated mammographies, 21.4 sonographies, 3.5 needle aspirations, 3.2 needle biopsies, 11.6 incisional biopsies, and 3.4 localization procedures. The respective costs of these procedures ranged from $40, $50, $150, $120, $155, $732, and $230 (low estimates) to $120, $120, $160, $300, $800, $2450, and $680 (high estimates). On the basis of this distribution of procedures and costs, the additional cost per abnormal mammographic result ranged from $157 to $439 (mean, $298).
The cost per screening mammography (based on an abnormal rate of 3%) ranged from $4.72 to $13.16 (mean, $8.94). Previous analyses assumed that follow-up procedures add $9 [20] to $11.50 [9] per mammography to the cost of a screening strategy. Our calculations approximate these assumptions.
The cost of screening mammography was derived from a population-based study. Because less than 3% of all mammographies are performed in mobile units, we considered only stationary units. The average charge (used as a proxy for cost) for a screening mammography was $91. Forty-one percent of stationary mammographic facilities did not distinguish between screening and diagnostic mammographies. These facilities charged a general fee of $118 (in 1992 U.S. dollars). Therefore, our use of $91 may bias the model toward screening. Although Medicare allowed a fee of $60 in 1992, most women who undergo screening mammography are too young for Medicare; this allowed fee may not reflect cost. Nevertheless, it was included in the range used in our sensitivity analysis. In British Columbia, a high-volume mobile mammography unit reduced the charge for a screening mammography to $40. Although it is not clear whether this reduction could be achieved on a large scale in the United States, this Figure was used as the lower bound in our sensitivity analyses.
Dr. Phillips: Prevention Sciences Group, 74 New Montgomery, San Francisco, CA 94102.
Author and Article Information
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References
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