WASHINGTON--A highly publicized journal article about the rate of
false positives in mammography distorts both the state of the field
and the goals and methods of breast cancer screening, said Edward A.
Sickles, MD, chief of the Breast Imaging Section, University of
California at San Francisco (UCSF).
Presenting the biennial Wendell Scott, MD, Memorial Lecture to a
plenary session of the American College of Radiologys 28th
National Conference on Breast Cancer, Dr. Sickles cited the
papers suggestion that new technologies are needed to reduce
false positives as an example of what happens "when
investigators write in areas outside their clinical expertise. They
Although technological progress is always welcome, he said, the
necessary methods for reducing false positives already exist and
"are not rocket science technology." Rather, they are
strategies for carefully using existing techniques.
But, lest the field pay too much attention to cutting the
false-positive rate, he cautioned that any method that reduces false
positives inevitably also results in some reduction in true-positive
cases. "It is very important that we understand that," he
said. "Do not react by immediately changing your thresholds of
sensitivity. Cancer detection, not false-positive rate, is the
measure of a screening program. If we wanted no false positives, we
Missing from the paper by Elmore et al (N Engl J Med
338:1089-1096, 1998), he said, is an understanding that in
mammography the definition of false positive varies according to the
With screening mammogram, a rating of 4 or 5 on the BI-RADS scale
indicates a possibly "positive" finding (ie, mammograms
that need more evaluation because they are suggestive or highly
suggestive of malignancy). This finding leads to a call-back for more
views that generally show no cancer. Thus, the original
"false-positive" finding resulted only in the need for more
noninvasive studies. At biopsy, on the other hand, a
false-positive finding would be a tissue study that did not indicate
pathology despite positive mammography findings. In this case, the
false-positive meant that the patient underwent an invasive procedure unnecessarily.
Dr. Sickles believes that radiologists can use several existing
methods to bring their false-positive level down as far as they
safely can. No mammography practice, however, should attempt to lower
its false-positive rate, he emphasized, "unless and until you
can demonstrate by rate of detection that the techniques used produce
acceptable sensitivity." Then and only then should attention go
to reducing false positives, he insisted.
An effective way of reducing recalls in screening is to compare a
patients current mammograms with previous films. Doing so cuts
the recall rate in half, he said. Women should be educated to bring
their old films with them when using a new mammography facility, he
Another cause of recalls is summation artifacts, which are harmless
objects photographically superimposed to resemble cancerous lesions.
Because they result from the perspective from which a particular view
is taken, these supposed lesions disappear when the breast is viewed
from another angle. Three percent of all exams produce such
single-view artifacts, which, in a moderately busy practice, can
occur as often as once a day. Taking two views at each examination
can therefore prevent needless call-backs, Dr. Sickles recommended.
Radiologists can also reduce recalls by electing to watch over time,
instead of immediately investigating, selected kinds of "doubtful
findings," Dr. Sickles said. Between 30% and 70% of cancers
found at screening are visible on previous films in "subtle
forms below the intervention threshold." If watched over time,
however, the great majority of such findings will prove to be benign,
A soon-to-be published study of 550 doubtful findings on mammography,
for example, found that over 2½ years of surveillance, 73% of
the lesions did not change, 20% went away, 3% became slightly more
prominent for technical reasons, and 1% were read as clearly abnormal
on a second exam.
Of these seven "abnormal" cases, three were shown to be
benign with additional workup, three were benign at biopsy, and one
was diagnosed as ductal carcinoma in situ (DCIS).
The strategy of watchful waiting of findings just below the
intervention threshold therefore produced results well within the
thresholds of avoiding malpractice, he noted.
In diagnosis, Dr. Sickles said, some summation artifacts will require
more than two views, but in this study, none of the findings that
looked suspicious on only a single view were found to be breast
Certain Findings Usually Benign
Certain kinds of findings, Dr. Sickles said, are "almost
certainly" benign, including circumscribed, noncalcified solid
masses. Ultrasound can ascertain for sure whether they are solid.
Findings that appear generalized over large areas of both breasts are
also likely to be benign. "The more of the same thing you have
scattered throughout both breasts, the less likely it is to be
cancer," he said.
By using follow-up, rather than biopsy, for patients with doubtful
findings, UCSF physicians achieve a biopsy yield of 38% rather than
the national average of 24%, Dr. Sickles said. He emphasized that
every practice should make vigorous efforts to calculate its own
biopsy yield (the ratio of positive biopsies to patients referred for
biopsies), to determine whether sensitivity is within an acceptable
By choosing follow-up instead of immediate tissue study in cases that
are probably benign, practices can reduce their rates of
false-positive mammograms in both screening and diagnosis without
changing their rate of finding early cancers, he said. This task
requires care and precision, but no new technology, he added.
"Everyone can do this," Dr. Sickles concluded.