Radiology: 3T MR is limited for breast cancer therapeutic response

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Although breast MRI can be used for surgical planning after neoadjuvant chemotherapy, diagnostic accuracy is uneven and is better in more aggressive than in less aggressive cancers, according to a study published in this month's Radiology.

Neoadjuvant chemotherapy (NAC) offers a number of benefits such as tumor shrinkage, improved operability and increased rate of breast-conserving surgery. “However, how to perform a successful breast-conservation surgery after NAC is challenging. It is difficult to determine how much tissue should be removed, especially in patients who responded well to the treatment,” wrote Jeon-Hor Chen, MD, from the Tu and Yuen Center for Functional Onco-Imaging and department of radiological sciences at University of California, Irvine, and colleagues.

The researchers sought to determine the diagnostic accuracy of 3T MR based on the molecular biomarker status of breast cancer among a cohort of women undergoing NAC. The current goal of NAC, according to the researchers, is to achieve pathologic complete response (pCR). Imaging may aid the decision-making process and inform therapeutic choice and the surgical plan.

However, 1.5T MRI, when used to evaluate NAC, has a high false-positive rate in cases where residual tumor scattered as multiple small foci over a large area. Chen and colleagues aimed to assess the utility of 3T MRI, which provides improved signal-to-noise ratio and spatial resolution, in assessing NAC response.

The study cohort included 50 women with biopsy-proven breast cancer who presented between November 2006 and October 2010.

All patients underwent a series of MR exams: at baseline, during therapy and after the NAC protocol. A breast MR specialist evaluated tumor response at the final MR study using subtraction images. A pathologist determined biomarker status, including human epidermal growth factor receptor (HER2), hormone receptors and the proliferation maker Ki-67. Results were correlated using Pearson correlation.

Among the 50 women, 14 achieved pCR, and there were seven false-negative diagnoses with MRI. All of the false negatives showed scattered small cancer foci at pathologic exam, wrote Chen and colleagues. Six of these cases were HER2-negative and one was HER2-positive.

These findings are similar to an earlier study using a 1.5T magnet, according to the researchers. “Therefore, the higher spatial resolution used at 3T did not substantially decrease the percentage of false-negative cases, and the results suggest that scattered residual cells or foci smaller than 2 mm are beyond the capability of MR imaging to depict.”

The overall sensitivity, specificity and accuracy of MRI for diagnosing post-NAC residual invasive disease were 81 percent, 93 percent and 84 percent, respectively.

Chen and colleagues reported strong correlation between MR imaging and pathologic tumor size when the analysis was narrowed to HER2-positive tumors. They suggested that women with HER2-negative hormone receptor-positive disease are the poorest candidates for MR evaluation.

The researchers called for additional studies to investigate the accuracy of MRI in different phenotypes and biomarker statuses, before concluding, “Breast MR imaging performed at 3T still has the same limitation compared with 1.5T for the detection of small tumor foci and scattered tumor cell clusters after NAC.” They also advised physicians to proceed with caution when using MR data in surgical planning among women with HER2-negative and hormone-receptor positive cancers and lesions with non-masslike enhancement as MR can underestimate the extent of residual disease in such cases.