Diagnostic imaging, oncology

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.

Professor Kuhl 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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