Radiology: PEM + MR improves breast cancer detection

Mediolateral oblique PEM image of 68 year-old woman, curved arrows indicate known cancer. Image courtesy of: Radiology.

The use of positron emission mammography (PEM) to complement MRI improves the detection of additional tumors in women expected to undergo breast-conserving surgery, according to a study to be published in the January edition of Radiology. The study found that PEM alone was less sensitive than MRI and less accurately predicted the need for mastectomy, though PEM boasted stronger specificity, making the modality less likely to prompt unnecessary biopsies.

Breast-conserving surgery followed by radiation is accepted as producing survival rates equivalent to mastectomy in women with breast cancer; however, mammography and clinical examination commonly underestimate the size of the cancer, requiring women to undergo re-excision or mastectomy after the initial breast-conserving surgery. MRI has been shown to have positive predictive value (of .69 in one study) for determining cancer size and in surgical planning, making the modality more effective than mammography, clinical examination and ultrasound for many pre-operative patients. Even so, the study pointed out, many tumors remain occult on all modalities, creating additional challenges for effective surgical planning and treatment of women with breast cancer.

The authors performed what they believed was the first direct comparison of PEM and MRI in ipsilateral breasts with newly diagnosed cancer. PEM and MRI were performed in randomized order in 388 women with invasive and/or intraductal breast cancer who were clinically eligible for breast-conserving surgery. PEM or MR readers were typically blinded to the findings of the other modality, though not to mammography and other clinical information, and when medically necessary physicians were informed of all of a patient's findings.

Of the 388 women with planned surgery, 82 were discovered to have ipsilateral cancer. Twenty-eight of these cancers were discovered by both PEM and MR imaging and 21 cancers were found by MRI alone. Fourteen additional cancers were discovered by PEM but missed by MRI. Seven cancers were discovered by mammography and ultrasound and twelve were missed by all imaging modalities utilized.

The integration of PEM with MR improved detection to 74 percent, compared with 60 percent for MRI alone. Fifty-six of the 388 participants ultimately required mastectomy, with MRI accurately identifying 40 of these women and PEM identifying 20.

"We found that PEM was more specific than MR imaging at the lesion level (151 [79.9 percent] compared with 124 [65.6 percent] of 189 benign lesions correctly characterized as imaging negative) and less likely to prompt unnecessary biopsies. At the lesion level, PEM was less sensitive than MR imaging (47 [41 percent] compared with 61 [53 percent] of 116 malignant lesions imaging positive), although there was no difference in sensitivity at the participant level," noted Wendie A. Berg, MD, PhD, of the American College of Radiology Imaging Network in Lutherville, Md., and colleagues.

PEM's mixed results led the authors to favor the modality as an addition to MRI and conventional imaging. "PEM proved to be complementary to MR imaging, increasing the additional ipsilateral cancer detection rate from 49 to 61 of 82 participants with additional cancer and depicting additional ipsilateral malignancies that were not seen on MR images in 14 of the 388 participants."

Several of the authors noted compensatory ties and funding for the present study from the PEM device-manufacturer and other companies.

"We found PEM sensitivity increased with increasing size of malignancy in this series," observed Berg and co-authors. "As such, the sensitivity of PEM for the detection of smaller lesions may be reduced in larger breasts owing to volume averaging," since all PEM scans incorporate 12 slices, contributing to less precision with greater volume.

PEM's positive predictive value of 66 percent was significantly higher than that of MRI's of 53 percent but lower than conventional imaging's 69 percent value.

"An important finding, however, was that even the combination of PEM and MR imaging did not fully depict the disease extent, particularly in cases with an extensive intraductal component, multifocal disease, or multicentric disease—that is, the very patient populations anticipated to benefit most from accurate preoperative assessment of disease extent."

Still, the authors concluded that "[r]eview of mammograms together with MR or PEM images, or both, improves the detection of additional disease."