Intraoperative magnetic resonance spectroscopy for identification of residual tumor during low-grade glioma surgery

Clinical article

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Object

The authors had previously shown that 3-T intraoperative MRI (ioMRI) detects residual tumor tissue during low-grade glioma and that it helps to increase the extent of resection. In a proportion of their cases, however, the ioMRI disclosed T2-hyperintense areas at the tumor resection border after the initial resection attempt and prompted a differential diagnosis between residual tumor and nontumoral changes. To guide this differential diagnosis the authors used intraoperative long-TE single-voxel proton MR spectroscopy (ioMRS) and tested the correlation of these findings with findings from pathological examination of resected tissue.

Methods

Patients who were undergoing surgery for hemispheric or insular WHO Grade II gliomas and were found to have T2 changes around the resection cavity at the initial ioMRI were prospectively examined with ioMRS and biopsies were taken from corresponding localizations. In 14 consecutive patients, the ioMRS diagnosis in 20 voxels of interest was tested against the histopathological diagnosis. Intraoperative diffusion-weighted imaging (ioDWI) was also performed, as a part of the routine imaging, to rule out surgically induced changes, which could also appear as T2 hyperintensity.

Results

Presence of tumor was documented in 14 (70%) of the 20 T2-hyperintense areas by histopathological examination. The sensitivity of ioMRS for identifying residual tumor was 85.7%, the specificity was 100%, the positive predictive value was 100%, and the negative predictive value was 75%. The specificity of ioDWI for surgically induced changes was high (100%), but the sensitivity was only 60%.

Conclusions

This is the first clinical series to indicate that ioMRS can be used to differentiate residual tumor from nontumoral changes around the resection cavity, with high sensitivity and specificity.

Abbreviations used in this paper:Cho = choline; Cr = creatine; DWI = diffusion-weighted imaging; GTR = gross-total resection; ioDWI = intraoperative DWI; ioMRI = intraoperative MRI; ioMRS = intraoperative long-TE single-voxel proton MR spectroscopy; LGG = low-grade glioma; NAA = N-acetyl-aspartate; VOI = voxel of interest.

Article Information

Address correspondence to: Koray Özduman, M.D., Department of Neurosurgery, Acıbadem University School of Medicine, Inönü Caddesi, Okur Sokak No. 20, Kozyatağı, Istanbul 34742, Turkey. email: koray.ozduman@superonline.com.

Please include this information when citing this paper: published online February 22, 2013; DOI: 10.3171/2013.1.JNS111561.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Newly designed plastic pins. The titanium pins of the original headrest/head coil combination were replaced by disposable plastic pins and used in this study to decrease susceptibility artifact (A). Diffusion tensor imaging, an echo planar–based MRI sequence that is very sensitive to and easily degraded by susceptibility artifact, was flawlessly performed both in the immediate preoperative setting with the patient's head fixed with pins (B) and in the intraoperative setting with the wound exposed to air (C).

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    Example of a case in which increased Cho/Cr ratio on ioMRS demonstrated residual tumor in an area that appeared normal on visual inspection: preoperative and intraoperative MR images with corresponding intraoperative photographs and ioDWI, ioMRS, and histopathological findings. This right frontal Grade II oligodendroglioma (A and D) was resected until no tumor was evident on visual inspection and intraoperative ultrasonography (E). The initial ioMRI study (B) showed a T2-hyperintense area at the posterior resection border (white arrows in B and E, labeled 1). Absence of diffusion restriction on DWI (G) and the presence of an increased Cho/Cr ratio on the long-TE spectrum (H) of the area identified by arrow 1 were suggestive of a tumor remnant, which was confirmed with histopathological examination (I). The patient underwent further resection of the area indicated on MRI (F) and a second ioMRI indicated total resection (black arrows in C and F, labeled 2). DWI and MRS of this normal-appearing area after the second resection showed no diffusion restriction (J) or increased Cho/Cr ratio (K), and examination of biopsy specimens obtained from the tumor border confirmed absence of tumor (L). NL = normal; non-restr = nonrestricted; res. = resection.

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    Residual tumor tissue and peritumoral changes may present similarly as T2-hyperintense areas, and MRS can help to differentiate between the two. This example demonstrates both residual tumor and peritumoral changes in the same case and demonstrates that a normal Cho/Cr ratio on MRS and absence of diffusion restriction on DWI will indicate absence of residual tumor. After resection of a right insular oligodendroglioma (A), a T2-hyperintense rim surrounding the resection cavity (B) was noted. Absence of diffusion restriction (C) and presence of a Cho peak on ioMRS (D) indicated residual tumor, which was confirmed histopathologically (E). In contrast to the expansile mass centered in the insula, the components in the frontal and temporal T2 hyperintensities had much less mass effect. After further resection a second set of ioMRI images was acquired, which revealed persistent T2 hyperintensities both in the temporal (F) and the frontal (K) lobe. Unlike the T2-hyperintense rim of the expansile insular mass, neither of these 2 areas had diffusion restriction (G and L, respectively) or exhibited increased Cho/Cr ratio on ioMRS (H and M, respectively). These ioMRI findings were suggestive of nonneoplastic nature, which was confirmed through histopathological examination of the respective biopsies (I and N, respectively). No growth was noted on either of these 2 areas at the 3-month follow-up MRI (J and O, respectively). NR = nonrestricted.

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    Example of a case in which surgically induced changes were identified by restricted diffusion on ioDWI. After removal of a right temporal astrocytoma (A), T2 hyperintensity posterior to the resection cavity was detected on ioMRI (arrow in B). Diffusion restriction was present at this T2-hyperintense region on DWI (C), indicating surgical manipulation–induced changes. A reversed lactate peak at 1.3 ppm supported the diagnosis of ischemic changes attributable to surgery. No increased Cho/Cr ratio was present on ioMRS (D). The T2-weighted images obtained at the 3-month follow-up (E and F) show encephalomalacia (F), supporting the diagnosis of surgical injury.

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    Correlation of ioMRS findings with biopsy findings in 20 patients.

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