Magnetic resonance imaging is better than multidetector computed tomography for determining if and how far breast cancer has invaded breast ducts and should be done before breast-conserving therapy, according to an article in the August issue of the American Journal of Roentgenology.

“Patients have a lower survival rate if their surgical margins are positive for tumor cells. A positive surgical margin is usually the result of inadequate resection of the cancer’s intraductal component,” said Akiko Shimauchi, MD, Tohoku University, Sendai, Japan. “Accurate preoperative diagnosis of the intraductal component allows the surgeon to achieve a cancer-free surgical margin.”

The study included 69 patients with proven invasive cancer, with 44 tumors known to have an intraductal component, said Shimauchi. Magnetic resonance imaging correctly identified 33 of the 44 cases, while multidetector computed tomography correctly identified 27 of 44.

“Magnetic resonance revealed the presence of the intraductal component with significantly higher sensitivity (75 percent) compared to multidetector computed tomography (61 percent),” Shimauchi said.

“The lesions that were missed by both examinations were the ductal extension type, that is--- the tumor included a dominant mass with an outward extension of cancer cells, with a relatively small ductal component,” said Shimauchi. Magnetic resonance imaging was better able to detect the smaller ductal components.
Although researchers found that both modalities “generally underestimated the length of the intraductal component,” magnetic resonance was less likely to underestimate the length of the intraductal component.

Shimauchi said, “In our institution, surgeons err on the side of caution by using a surgical margin that is 20 mm outside the radiologically determined tumor margin.”

She added that underestimation of the length of the intraductal components by 15 mm or more was significantly less frequent with magnetic resonance (30 percent) than with computed tomography (55% percent).