Whole-body MRI falls short in detecting distant metastases in kids

Whole-body MRI is not justified as the initial study for staging of pediatric malignant tumors, with accuracy no better than conventional imaging for detection of distant metastatic disease, according to a study published in the February issue of Radiology.

While whole-body MRI was able to detect more skeletal lesions, on average, than conventional imaging, it falls short of conventional imaging for detection of tumors that have spread to the lungs and solid organs, reported Marilyn J. Siegel, MD, of Washington University School of Medicine, St Louis, and colleagues.

Because many techniques for the staging and management of pediatric solid tumors require exposure to ionizing radiation, the authors sought to evaluate MRI because “alternative imaging methods that do not use ionizing radiation but allow similar diagnostic accuracy would be particularly attractive.”

Siegel and colleagues conducted a multicenter, prospective cohort study that included 188 patients with a mean age of 10.2 years. Patients had newly diagnosed lymphoma, neuroblastoma or soft-tissue sarcoma. All patients underwent noncontrast whole-body MRI in addition to standard-practice conventional imaging, which included some combination of CT, scintigraphy or FDG PET, depending on the patient.

A total of 66 patients—33 positive and 33 negative metastasis—were ultimately selected for image review and analysis. Results showed that MRI did not meet the noninferiority criterion for accuracy when compared with conventional imaging for detection of metastasis, with a difference between average area under the receiver operating characteristic curve of -0.03.

“However, whole-body MR imaging was significantly more accurate for the detection of distant metastatic deposits in solid tumors than in the detection of lymphomas,” wrote the authors.

Among 19 patients with distant skeletal disease, whole-body MRI allowed detection of 5.9 regions of disease per patient compared with 3.7 regions per patient with conventional imaging. “These results are in agreement with prior studies that have shown that whole-body MR imaging with [short tau inversion-recovery] was superior to bone scintigraphy for detection of skeletal metastases,” added Siegel et al.

It was in the detection of extraskeletal disease that whole-body MRI showed its limitations. “In our study, there were eight patients with positive findings at conventional imaging whom most of the readers found to be negative for metastasis on the basis of whole-body MR imaging,” wrote the authors. They pointed out that six of the eight missed lesions were 1 cm or less in diameter and all were located in the lung or liver.

“The understaging of pulmonary and hepatic disease on the basis of whole-body MR imaging is not surprising and has been shown by other investigators,” wrote the authors. “Whole-body MR imaging can help detection of relatively large pulmonary nodules, but smaller lesions (< 10 mm in diameter) may go undetected.”