MR, nuclear medicine show advantages over mammography

Competitors to x-ray mammography may be mounting a serious challenge to the long-standing technology. Research has determined that MR and nuclear medicine can detect ductal carcinoma in situ (DCIS), which can develop into an invasive form of breast cancer, much earlier than mammography can.

MR may be especially attractive as the modality shows promise as a screening tool for patients not typically considered high risk. Early detection of DCIS substantially improves the odds of patient survival, as the cancer in this stage is unlikely to have spread to other parts of the body.

"If we find DCIS and remove it, we can prevent the formation of 'real' breast cancer," said Christiane Kuhl, radiology professor at the University of Bonn in Germany. "That way we prevent the development of a disease that is often life-threatening."

In this form of cancer, malignant cells grow inside the milk ducts, where the tumor takes shape. At this stage, the tumor or carcinoma can be successfully removed by surgery. It presents a very serious threat, however, if it grows out of the milk ducts into the breast's glandular tissue from which it can spread via blood and lymph vessels in the body.

Although MRI had been thought to be ill-suited to the detection of DCIS, research at the University of Bonn proved otherwise. Over the past five years, Kuhl and colleagues examined more than 7000 women with both MR and mammography. Their examinations detected early forms of DCIS in 167 women. Of these, 152 (92 %) were found using MR, whereas just 93 (56 %) were found with mammography.

DCIS might also be detected using a form of nuclear medicine. Dr. Rachel Brem, director of breast imaging and intervention at George Washington University Medical Center found that breast-specific gamma imaging (BSGI) can reliably detect very small DCIS lesions, providing higher sensitivity than mammography or MR. Brem and colleagues evaluated the procedure on women with mammographically suspicious microcalicifications and other high-risk factors

BSGI detected all four DCIS lesions smaller than or equal to 5 mm and all six DCIS lesions less than or equal to 10 mm with measurable residual disease at surgical excision. The study examined 20 women with 22 biopsy-proven DCIS lesions. Overall, BSGI demonstrated 91% sensitivity for DCIS, specifically detected low-grade DCIS, and identified several lesions not found on mammography or MRI. The smallest lesion noted was 2 mm.

Finding DCIS can be a matter of life and death. Low-grade DCIS is relatively inert. If it remains in the milk duct, the carcinoma presents little risk to the patient. High-grade DCIS, however, typically spreads outside the duct, presenting a very serious danger.

Mammography visualizes small calcifications that form in the affected ducts. These calcifications are not visible on MR, which is why the modality was not believed appropriate for breast cancer screening. The Bonn researchers found, however, other signs of DCIS, specifically changes in contrast enhancement, which appeared on MR much earlier than calcifications could be seen on mammography.

The argument for breast MR as a screening method gains added weight when considering the increased sensitivity of the modality to the most aggressive form of this carcinoma.

"The particularly aggressive high-grade DCIS was especially reliably picked up using MRI, but especially difficult to detect using mammography," Kuhl said.

The Bonn researchers found 89 cases of high-grade DCIS. All but two were detected using MR, whereas mammography missed 43.

The fast-growing tumors do not develop the calcifications, according to Kuhl. Instead, these DCIS lesions are perfused by many small blood vessels, which produce a detectable signature on contrast-enhanced MR.

The use of MR to spot DCIS at the University of Bonn was not plagued by the false positives that usually are cited as another of its shortcomings. MR, in fact, demonstrated a positive predictive value of 59% compared with 55% for mammography.

MR came out ahead of mammography at George Washington University Medical Center, but the best results were obtained with a high-resolution, small field-of-view gamma camera providing craniocaudal and mediolateral oblique projections.

The findings indicate that the pathologic tumor size of the DCIS ranged from 2 to 21 mm. Of the 22 cases of biopsy-proven DCIS in 20 women, 91% were detected with BSGI, 88% with MR, and 82% with mammography. Two cases (9%) were diagnosed only after BSGI demonstrated an occult focus of radiotracer uptake in the contralateral breast, previously undetected by mammography. There were two false-negative BSGI studies.

The Bonn research team concluded that MR can improve significantly the diagnosis of very early stages of breast cancer in all women, not simply high-risk groups. Its success, however, should not come at the expense of mammography, according to Kuhl.

"In order to find these preinvasive stages of breast cancer on an MR scan, the images need to be evaluated according to specific criteria," she said. "Unfortunately, awareness of this is not yet sufficiently widespread."