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Lisa M Schwartz a Veterans Administration
Outcomes Group (111B), Veterans Administration Medical Center, White
River Junction, VT 05009, USA, b Department of
Medicine, Dartmouth-Hitchock Medical Center, Lebanon, NH 03756, USA, c Department of Social and Decision Sciences, Carnegie Mellon
University, Pittsburgh, PA 15213, USA
Correspondence to:
L M Schwartz lisa.schwartz{at}dartmouth.edu
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Abstract |
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Objective:
To determine women's attitudes to and
knowledge of both false positive mammography results and the detection
of ductal carcinoma in situ after screening mammography.
Screening mammography is vigorously promoted in the United States.
With the exception of the US government's Preventive Health Services
Task Force, professional organisations recommend that women begin
annual or biannual screening at 40 years of age.1-3 Mammography is promulgated by hospitals, insurance plans, and breast
care centres. Efforts for quality improvement commonly focus on
increasing the screening rates for breast cancer, and health plans
highlight these rates on cards used for reporting quality of health
care. Although there has been much discussion about the potential
benefit of mammography there has been far less about the potential harms.
The harm that has received the most attention is false positive
results. Mammograms that give false positive results are common. A 60 year old woman screened annually for 10 years has a 50% chance of
having at least one false positive leading to follow up testing and a
20% chance of a false positive leading to biopsy.4
Consequently, many people are concerned about the physical,
psychological, and economic costs of false positives.4-10
Several experts in screening have concluded that women would benefit
from education about false positive results if they are to make
informed decisions about whether to undergo, or continue with,
screening.
2 11-13
Little attention has been paid to the increasingly frequent detection
of ductal carcinoma in situ, a subtle but potential harm of
screening.14 Although the clinical course of ductal carcinoma in situ is poorly understood, most lesions do not
progress.14-17 Consequently, an increasing number of
women with lesions that would never have become clinically apparent are
worried about cancer, and most of them will undergo invasive treatment
of unknown benefit (for example, mastectomy, lumpectomy with
radiation).14
It is not known if or how women who are offered screening are being
counselled about false positives and ductal carcinoma in situ. To
determine what women know we conducted a national survey of women in
the United States, a population with high exposure to mammography Design
Design:
Cross sectional survey.
Setting:
United States.
Participants:
479 women aged 18-97 years who did not
report a history of breast cancer.
Main outcome measures:
Attitudes to and knowledge of
false positive results and the detection of ductal carcinoma in situ
after screening mammography.
Results:
Women were aware that false positive results do occur. Their median estimate of the false positive rate for 10 years
of annual screening was 20% (25th percentile estimate, 10%; 75th
percentile estimate, 45%). The women were highly tolerant of false
positives: 63% thought that 500 or more false positives per life saved
was reasonable and 37% would tolerate 10 000 or more. Women who had
had a false positive result (n=76) expressed the same high tolerance:
39% would tolerate 10 000 or more false positives. 62% of women did
not want to take false positive results into account when deciding
about screening. Only 8% of women thought that mammography could harm
a woman without breast cancer, and 94% doubted the possibility of
non-progressive breast cancers. Few had heard about ductal carcinoma in
situ, a cancer that may not progress, but when informed, 60% of women
wanted to take into account the possibility of it being detected when
deciding about screening.
Conclusions:
Women are aware of false positives and
seem to view them as an acceptable consequence of screening
mammography. In contrast, most women are unaware that screening can
detect cancers that may never progress but feel that such information would be relevant. Education should perhaps focus less on false positives and more on the less familiar outcome of detection of ductal
carcinoma in situ.
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Introduction
Top
Abstract
Introduction
Participants and methods
Results
Discussion
References
more
than 85% of US women aged 40 years or more have had at least one
screening mammography.18 We wanted to find out if women
are aware of false positives and if they have a sense of the chance of
having one; if false positives are tolerated because women have an
unrealistic sense of the benefit of mammography; and if women are aware
of ductal carcinoma in situ and, if not, whether they want to know
about it.
![]()
Participants and methods
Top
Abstract
Introduction
Participants and methods
Results
Discussion
References
We randomly selected women from details compiled from telephone
directories and administrative records (for example, applications for a
driver's licence, electoral registries, house purchases) by National
Decision Systems (Atlanta, GA). We restricted our sample to the 80% of
US women in households with telephones. We used stratified random
sampling to oversample women of screening age. Specifically, we
selected women by age (18-39 years old, 200 women; 40-49, 250; 50-69, 250; 70 or older, 100), estimated income (income more or less than
twice the 1992 poverty threshold for a family of four
people19), and area of residence.
Survey
We developed a 13 page questionnaire as part of a larger project
on women's decision making about mammography. A pilot was tested on
women veterans served by the Veterans Administration Medical Center at
White River Junction, Vermont.
Visual analogue scales
We asked women to estimate the sensitivity and false positive rate
of mammography with a previously validated visual analogue
scale.20 To familiarise respondents with the scale, we
included two practice questions about events having extreme
probabilities. Overall, 94% of respondents used the correct end of the
scale for each event
close to 1 for the chance of stopping at a red
light, and close to 0 for the chance of being hit by a meteorite.
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Analysis
Because we used stratified random sampling, we calculated sample
weights to account for probability of selection and to compensate for
small differences in response rates across sample strata. We then
adjusted the sample distribution to conform to known marginal
distributions of the US population based on data from the 1990 US
census
19 21
by creating "balance
weights."22 Because the crude results and the weighted
results were almost identical, for simplicity we present the crude
data. Based on our sample size, we estimate the margin of error of the
results to be 4-6% in either direction.23 All analyses
were done with STATA software (College Station, TX).
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Results |
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Sample characteristics
Table 1 shows the characteristics of the women. Respondents were
from all 50 states and the District of Columbia. Most women reported
having had at least one mammogram: 35% of women less than 40 years of
age, 87% of women in their 40s, 93% of women aged 50-69, and 87% of
women aged 70 or more.3 Similarly high proportions of
women planned to have a mammogram in the next two years.
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Perception of harm
Overall, 441 (92.0%) women believed that mammography could
not harm a woman without breast cancer (table 2). Thirty of the 40 women who thought harm was possible responded to our request for an
explanation. The most common responses were exposure to radiation (16 women), stress or anxiety (four), and false positives (three). None
mentioned the effects of treating non-progressive cancer.
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False positive results
Overall, 99% of women believed that false positive results
occur during a 10 year programme of annual mammography beginning at age
60 years. The women's median estimated chance of a false positive
during such a programme was 20%. This estimate is in line with a
recent report citing a 47% 10 year probability of a false positive
mammogram leading to any follow up testing for a 60 year old woman and
a 19% probability of a false positive mammogram leading to a
biopsy.4
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that is, a breast biopsy but no
diagnosis of breast cancer. In this subgroup, 93% believed that
mammography could not harm a woman who turned out not to have breast
cancer, 35% wanted to take false positives into acount when deciding
on screening, 71% would tolerate 500 or more false positives per life
saved, and 39% would tolerate 10 000 or more (fig 2).
Perception of benefit
To explore whether this high tolerance reflected an unrealistic
sense of the benefit of mammography, we examined perceptions of
benefit. As expected, most (94%) of the women believed that women
whose breast cancer was diagnosed by screening mammography benefited
from having been screened (table 2). Although most believed that
mammography reduced the chance of dying of breast cancer, none thought
it reduced the risk to zero. The most common expectation was that
mammography would reduce the chance of dying of breast cancer by half
and the second most common expectation was that it would reduce the
chance by one third
24 25
(we considered this to be the
correct answer; however, a recent study suggests that a one third
reduction in risk may be an overestimate26). Women were
aware that mammograms did not find all cancers. Their median estimated
sensitivity for a single mammogram (for a 60 year old woman) of 73%
underestimated the reported sensitivity of 94% (95% confidence
interval 83% to 99%).27
Non-progressive cancer
Few women knew about the possibility of non-progressive breast
cancer (table 2). Only 7% agreed that some breast cancers grow so
slowly that even without treatment they would not affect a woman's
health. We gave the following brief explanation of ductal carcinoma in
situ: "We would like to ask your opinion about ductal carcinoma in
situ or DCIS, a breast abnormality which can only be picked up by
mammograms. Cancer specialists are confused about DCIS because
sometimes it becomes invasive and sometimes it doesn't. If DCIS does
not become invasive, it will not affect how long a person will live
even without treatment. Doctors don't know which DCIS will become
invasive. Nowadays, almost everyone with DCIS gets treated. Many people
receive surgery, chemotherapy, or radiation who would never have gotten
sick. For these people, treatment provides no physical benefit."
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Discussion |
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The women in our study were aware of false positive results from screening mammography but seemed to view them as an acceptable consequence of screening. Although studies attest to the short term physical and psychological impact of false positive results,5-9 our respondents were highly accepting of them: most would not take them into account when deciding about screening, and almost 40% would tolerate 10 000 or more false positives requiring biopsy for each life saved.
One explanation for this high tolerance is that women have an overly optimistic sense of the benefit of mammography. We found no evidence to support this explanation. No respondent thought screening mammography eliminated the chance of dying of breast cancer. Women were aware that mammography misses some cancers (actually underestimating the reported sensitivity). Most women also recognised that health promoting behaviours such as not smoking, exercising regularly, and eating a low fat diet were more beneficial than mammography in prolonging life, which is true for the average 60 year old woman.28
Alternatively, it might be posited that women did not fully understand the consequences of a false positive result (for example, anxiety, pain, inconvenience, or extremely rare harms such as severe infection or death related to anaesthesia). We found, however, that women who had had false mammograms expressed the same high tolerance as women who had not. Thus, women seemed to think that false positives are worth the reassurance of being told they do not have cancer. Similarly, Gram et al found that almost half of women with false positive mammograms viewed the experience as having an overall positive impact on their lives,8 and most women continue to undergo mammography.29
Women's perceptions about a potential diagnosis of ductal carcinoma in situ differed noticeably from their perceptions about false positive mammograms. In contrast to false positives, most women were unaware of non-progressive forms of breast cancer and even doubted their existence. Once informed about non-progressive cancer, the women seemed concerned. Most wanted to take into account the possibility of ductal carcinoma in situ when deciding about screening. Younger women, in whom 90% of the cancers found by screening mammography are ductal carcinoma in situ,15 were the most interested in such information. In addition, women's reported willingness to treat ductal carcinoma in situ increased as we hypothetically increased the chance of ductal carcinoma in situ progressing to invasive breast cancer, suggesting that such information might influence decisions.
Limitations
One limitation of our study is the representativeness of the
sample: we did not include women in households without a telephone and
those who requested that their name be removed from the database. This
left about 80% of US women eligible for sampling. Secondly, although
our sample represents women across a broad range of age, education, and
income, it differed from the general population: the women were
wealthier and better educated, and almost all were white. Women from
ethnic minorities and women with the lowest socioeconomic
indicators were underrepresented. Subsequent studies are needed to
assess whether such women have different perceptions.
that is, that false positives are part of screening. In
contrast, more time should probably be spent educating women about the
less familiar outcome of the ambiguity associated with the detection of
ductal carcinoma in situ.
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What is already known on this topic
False positive results and diagnoses of non-progressive cancer are recognised problems of screening mammography Little is known about how women feel about these problems What this study addsAlmost all of the 479 women (99%) knew that false positive mammograms occur Women do not seem to think that false positive mammograms are an
important harm of screening Women's tolerance of false positives is not explained by overly optimistic beliefs about the benefit of mammography Few of the women (6%) were aware of the possibility of non-progressive cancer |
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Acknowledgments |
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We thank Robert Pritchard for help with the study design, R Peter Mogielnicki for helpful comments, and Jennifer Winder and Suzy Shukwit for assistance with data collection.
Contributors: LMS and SW collaborate on all work and are the joint principal investigators on the Department of Defense grant "enhancing informed decision making about mammography"; they will act as guarantors for the paper. They initiated and coordinated the formulation of the study hypotheses, discussed core ideas, designed the study protocol, developed the survey, and participated in the data collection, analysis, and writing of the paper. HCS and BF participated in formulating the study hypotheses, discussing core ideas, designing the study, developing the survey, and writing the paper. HGW participated in the formulation of study hypotheses, discussing core ideas, designing the study, developing the survey, the analysis, and writing of the paper.
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Footnotes |
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Funding: SW and LMS are supported by the veterans affairs career development awards in health services research and development. This work was also funded by a new investigator award from the Department of Defense breast cancer research program (DAMD17-96-MM-6712). The views expressed do not necessarily represent those of the Department of Veterans Affairs or the US government.
Competing interests: None declared.
This article is part of the BMJ's
randomised controlled trial of open peer review. Documentation relating
to the editorial decision making process is available on the BMJ's
website
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References |
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(Accepted 15 March 2000)
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