MRI beats mammography in detecting early-stage breast cancer
21 August 2007
A study by researchers at the University of Bonn has
shown that magnetic resonance imaging (MRI) is more accurate than
mammography in diagnosing very early stages of breast cancer. Up to now MRI
was thought to be hardly suited for the detection of early stage breast
cancer. The study was published in the 11 August issue of The Lancet.
Mammography is considered the 'gold standard' of breast cancer medicine, but
it has been known for a long time that MRI is superior to mammography as far
as the diagnosis of invasive breast cancer is concerned.
However, the search for 'ductal carcinoma in situ' (DCIS) has been the
preserve of mammography. It highlights small calcifications which form in
the milk ducts affected. On the MRI scan, depositions of this kind are
invisible, therefore MRI was hitherto seen as unsuited for detecting
intraductal carcinoma, which was one of the main reasons for only using
mammography for the early detection of breast cancer.
Breast cancer forms
in the cells which line the inside walls of the milk ducts. The tumour
initially remains in the milk duct and is therefore called "intraductal
cancer" or DCIS (ductal carcinoma in situ). Although even at this stage it
is called a carcinoma, it behaves like a benign disease, which can always be
treated successfully by operating on it. Only when the tumour grows out of
the milk ducts into the breast’s glandular tissue can it spread via blood
and lymph vessels in the body. "If we find DCIS and remove it we can prevent
the formation of 'real' breast cancer," the Bonn radiologist Professor
Christiane Kuhl explains. "That way we prevent the development of a disease
that is often life-threatening."
However, there are different forms of
intraductal carcinomas: the less aggressive (low-grade) and highly
aggressive (high-grade) DCIS. Whereas low-grade DCIS is relatively inert and
may never spread beyond the milk duct (and thus may never pose a threat to
the women affected), high-grade DCIS will virtually always break out of the
milk ducts, and then will become dangerous, biologically aggressive high
grade invasive breast cancer. This makes it all the more important to
discover such high-grade DCIS.
A total of 7319 women were investigated by
Professor Kuhl and her colleagues over five years using both methods. In 167
of them they found early stages of breast cancer. Mammography only detected
93 cases of DCIS, compared with 153 cases detected by MRI. "And not only
that: it was above all the particularly aggressive high-grade DCIS which
were especially reliably picked up using MRI, but especially difficult to
detect using mammography," said Professor Kuhl.
A total of 89 cases of
high-grade DCIS were discovered by the doctors in the course of the study.
MRI, detected 98% of these aggressive pre-invasive breast cancers, while
mammography only detected 52%. The reason for this was that as it appears
that the fast-growing tumours do not develop the calcifications which
constitute the basis of mammographic DCIS diagnosis. Instead, these DCIS are
pervaded by many small blood vessels in which the contrast medium that is
injected for an MRI scan collects particularly well.
concludes from this that "Our study demolishes a whole series of textbook
dogmas. Firstly, it was always said that MRI was not able to find early
stages of breast cancer in the milk ducts. As our results show, the opposite
is true. MRI is far more sensitive than mammography."
The second prejudice is that MRI often leads to 'false positives', ie
often causes false alarms. "Among our patients that was even less the case
with MRI than with mammography," Professor Kuhl emphasises. "The positive
predictive value of MRI was 59%, compared to 55% for mammography."
The third point was that until now it was assumed that MRI was only
necessary for early detection of breast cancer in women with an increased
risk of breast cancer. Yet many of the participants had taken part in the
study as part of the perfectly normal cancer check-up. The conclusion of the
Bonn research team is that MRI can improve the diagnosis of very early
stages of breast cancer significantly, not simply with high-risk groups, but
with all women.
More expertise needed
However, Professor Kuhl
does not want the Bonn results to be used as an argument against current
mammography screening programmes. "Mammography is a basic examination that
is essential for early detection." MRI is not yet ready for use as a mass
screening tool. "Too little use is made of the method in the field of breast
imaging, so correspondingly there are too few radiologists who have been
able to gain sufficient experience with this specific application," she
says. "In order to evaluate the MRI scans, an experienced eye is needed. It
is the radiologist that diagnoses the patient, not the magnet. In order to
find these pre-invasive stages of breast cancer on an MRI scan, the images
need to be evaluated according to specific criteria. Unfortunately awareness
of this is not yet sufficiently widespread."
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