It has been reported that up to 25% of lesions are missed by mammography. Computer-aided detection can help improve the sensitivity of breast cancer diagnosis and the consistency of radiologists interpretation of mammograms. This article presents the results of a study comparing the performance of ImageChecker M1000 and SecondLook systems in the detection of biopsy-proven breast cancer
is the Medical Director and
are Staff Radiologists specializing in mammography at the Jane
Brattain Breast Center, Park Nicollet Clinic, Minneapolis,
Breast cancer is one of the leading causes of cancer death in
Early diagnosis of breast cancer plays an essential role in
reducing mortality rates and improving patient prognosis.
Currently, mammography is the gold standard technique for screening
detection of nonpalpable malignancies.
However, this method has some limitations, as some studies indicate
that up to 25% of lesions are not detected by mammography.
Mammographic interpretation can be improved when a second
radiologist reads the mammogram (known as double reading),
increasing breast cancer detection by up to 15%.
However, double reading is often not achievable and is not
currently advocated as a standard of care.
Therefore, further improvement in the sensitivity of screening
mammography is needed.
Such improvements have been made possible with the development
of computer-aided detection (CAD).
This emerging technology can help to improve the sensitivity of
breast cancer diagnosis and the consistency of radiologist
interpretation of mammograms.
Clinical studies have shown that CAD increases radiologist
sensitivity for the detection of breast cancer by up to 20%,
without increasing the work-up rate or the radiologist reading time
(hence, radiologist workload).
This indicates that CAD may help to reduce the rate of missed
cancers in the context of screening mammography and, potentially,
may provide clinical benefits.
Three CAD systems are now commercially available to the
radiologist: ImageChecker M1000 (R2 Technology, Los Altos, CA),
Mammoreader (Intelligent Systems Software, Clearwater, FL), and
Second Look (CADx Medical Systems, Montreal, Quebec, Canada). In
practice, the radiologist uses CAD after the initial review of the
mammograms. CAD systems are designed to provide the radiologist
with visual prompts in specific regions on the image.
The films are processed by the CAD system, which uses proprietary
algorithms to detect potential areas of concern and highlight
potentially suspicious areas (such as masses, microcalcifications,
architectural distortions, and asymmetric densities). With the
information provided by the CAD system, the radiologist decides
whether the highlighted regions of concern warrant work-up and
retains the ability to make the final diagnosis. These three CAD
systems therefore represent second opinion readers that can assist
the radiologist by identifying areas that may have otherwise been
This study was undertaken in October 2001 to compare the
performance of ImageChecker M1000 and Second Look in the detection
of biopsy-proven breast cancer. The sensitivity of these two CAD
systems for the detection of the breast cancer lesions was
Materials and methods
The mammograms used in this study were obtained from the Jane
Brattain Breast Center, Park Nicollet Clinic (Minneapolis, MN).
Three experienced mammographers participated in the study. They
randomly selected 120 biopsy-proven breast cancer cases. The
mammograms that led to the diagnosis of each case were reviewed by
both CAD systems. There were 126 lesions identified on the 120
mammograms, which consisted of two types of projection images:
craniocaudal (CC) and mediolateral oblique (MLO) views. The lesions
found on the mammograms included 69 masses, 47 microcalcifications,
and 10 mixed mass/microcalcification lesions. Of the 120 cases of
breast cancer found, in 67 cases the lesion was predominantly a
mass, 43 had microcalcifications predominantly; 10 cases had mixed
mass/microcalcification lesions. The cancers were of various types
and grades and included ductal carcinoma in situ (DCIS) and
invasive carcinoma (ductal and lobular). The sensitivity of the
systems was assessed from the percentage of cancer cases identified
correctly. A case was considered true positive if cancer was
identified by CAD in at least one view. The number of marks per
film made by the CAD systems was also determined.
Image Checker system
The ImageChecker M1000 system consists of a laser digitizer, a
computer using proprietary signal-processing algorithms, and a
customized motorized viewer with video display monitors.
ImageChecker M1000 digitizes the mammograms at high resolution (50
µm pixels) and 12 bits of gray scale (4096 shades of gray). The
processing algorithm searches for areas of microcalcifications and
masses or architectural distortions. ImageChecker M1000 marks
microcalcifications with solid triangles and highlights masses or
architectural distortions with asterisks.
Second Look system
The Second Look CAD system is a self-contained unit that
includes a high-resolution digitizer, a computer running
proprietary algorithms, a printer, and a touch-screen monitor. The
system digitizes the mammograms at high resolution (43.5 µm) and 12
bits of gray scale (4096 shades of gray). The proprietary
algorithms search for areas of microcalcifications and masses
(spiculated and nonspiculated, asymmetric densities, and
architectural distortions) prior to the generation of a paper
report, known as the Mammagraph. Second Look marks
microcalcifications with rectangles and suspicious masses with
The radiologist reviews the CAD results and takes a second look
at the films in the locations of the potential regions of concern
detected by CAD, to determine whether these suspicious areas
Comparison of the CAD systems
The performance of each system was compared with the other by
assessing the sensitivity and specificity of breast cancer
detection for the ImageChecker and Second Look systems.
Analysis of the performance of both CAD systems, based on the
total number of lesions, revealed high sensitivity for the
detection of breast cancer (Table 1). ImageChecker M1000 correctly
marked 114 (90.5%, 95% CI: 84 to 96) and Second Look correctly
marked 111 (88.1%, 95% CI: 81 to 93) of the126 lesions (Table
Both CAD systems showed a greater sensitivity for the detection
of microcalcifications than for masses. ImageChecker M1000
correctly marked 100% of microcalcifications, while Second Look
correctly identified 93.6%. Second Look correctly marked 82.6% of
the mass lesions and identified 100% of the mixed
mass/microcalcification lesions. ImageChecker M1000 highlighted
84.1% of the mass lesions and correctly marked 90% of the mixed
Both CAD systems demonstrated a high sensitivity for the
detection of breast cancer cases (Table 2). ImageChecker M1000
correctly marked 108 (90%, 95% CI: 83 to 95) of the 120 cases of
breast cancer, while Second Look correctly identified 107 (89.2%,
95% CI: 82 to 94) of them (figure 1). Second Look detected 97.7% of
the cases of microcalcifications and ImageChecker M1000 marked
100%. ImageChecker M1000 highlighted 83.6% of the cases including
masses and Second Look identified 82.1%. While ImageChecker M1000
correctly marked 90% of the mixed mass/microcalcification lesions,
Second Look identified 100%.
Characteristics of cancers missed by CAD
A subgroup of 6 breast cancer cases was not identified by either
of the CAD systems (Table 3). There were 9 cases that were
identified correctly by ImageChecker M1000, but not by Second Look
and 6 cases marked correctly by Second Look, but not by
ImageChecker M1000. These subgroups of missed cases consisted of 19
invasive ductal and lobular cancers and 2 cases of DCIS with
Distribution of marks per case
The distribution of marks made by the CAD systems is illustrated
in Figure 2. Four films were used for each case. The number of
Second Look marks per case ranged from 0 to 7, with an average of
1.33 marks/film. For ImageChecker M1000, the range of marks per
case was 0 to 12, with an average of 1.23 marks per film.
Statistical analysis of the CAD systems' performance
The sensitivities of the CAD systems were compared with a
: Second Look and ImageChecker M1000 have the same sensitivity in
detecting breast cancer in at least one view.
: The sensitivity of these CAD systems are different in their
ability to detect breast cancer in at least one view.
To reject the null hypothesis with a 90% confidence interval
requires a z score of 1.645. Comparisons of the case and lesion
sensitivities of ImageChecker M1000 and Second Look give: z case =
0.2113; z lesion = 0.616. Therefore, this result supports the
statement that ImageChecker M1000 and Second Look have equivalent
sensitivity in detecting screen-detected breast cancers in at least
This study is the first head-to-head comparison of the
performance of two commercially available CAD systems, ImageChecker
M1000 and Second Look, on the same breast cancer cases and at the
same time. Second Look and ImageChecker M1000 showed a similar high
sensitivity for the detection of breast cancer in at least one
view. The results obtained in the present study are consistent with
those of previous clinical studies separately assessing the
sensitivity of ImageChecker M1000
and of Second Look.
It has been documented that mammography can reduce mortality by
But, despite the recognized usefulness of mammography, the chance
that breast cancer can be missed with this method can be as high as
25%. It has been suggested that up to two-thirds of the cancers
missed by mammography reading are potentially detectable.
Therefore, it is evident that an improvement in mammographic
sensitivity is needed.
Some studies have shown that the detection of breast cancer may
increase by up to 15% when a second radiologist interprets the
However, the practice of double reading has not been widely
instituted and is not currently advocated as a standard of care.
Advances in computer technology have led to the development of
CAD systems that can serve as second readers to assist radiologists
in the clinical setting.
With CAD systems, radiologists have a means to overcome the known
limitations of human interpretation of mammograms. Studies have
shown that CAD significantly increases radiologist sensitivity for
the detection of breast cancer, without increasing the
The present study demonstrated that ImageChecker M1000 and Second
Look have a high detection rate for features of biopsy-proven
breast cancer on mammograms.
This study was designed to consider one-view detection a true
positive. It should be stated, however, that two-view detection
would reflect the sensitivity of a CAD system more accurately.
Radiologists must be aware that true abnormalities, including
cancers, may be marked by a CAD system on only one view. There were
14 cancer cases identified correctly by ImageChecker M1000 in two
views and by Second Look in one view. On the other hand, 6 cases
were identified correctly by Second Look in two views and by
ImageChecker M1000 in one view.
The results of this study indicate that CAD systems may help to
reduce the rate of missed breast cancers. The two CAD systems
evaluated in this study, ImageChecker M1000 and Second Look, have
demonstrated an equivalent performance in their sensitivity of
detection of breast cancers when using one mammographic view as the
criterion for a positive finding.