Postoperative Evaluation

The differentiation of tumor recurrence from radiation necrosis in patients with malignant gliomas who have been treated previously remains a challenge. Multiple imaging modalities were evaluated to address the problem, including FDG-PET, MRS, and perfusion imaging, in view of the limitations of conventional MRI in these cases. FDG-PET has been traditionally used in the differentiation of recurrence from radionecrosis. Sensitivities for detection of recurrent tumor ranged from as low as 43% to as high as 86%.61,62,77,78 Lower specificity values, however, were found, and most investigators considered the modality insufficient for the evaluation of tumor recurrence.62,77

MRS/I is finding a niche in therapeutic monitoring and has recently been applied with success to the classic problem of differentiating recurrent tumor from radiation injury. In one study (n = 56), a significant difference in metabolite ratios (Cho/Cr and Cho/NAA) emerged between neoplastic and nonneoplastic lesions after stereotactic radiotherapy.79 In another prospective study involving 27 patients, MRSI spectral patterns allowed reliable differential diagnostic statements to be made when the tissues are composed of either pure tumor or pure necrosis; however, these were less definitive in tissues composed of varying degrees of mixed tumor and necrosis.80 Perfusion imaging is also helpful in the prognostic evaluation and postoperative follow-up of patients with brain tumors. Although tumors that recur after radiation therapy tend toward lower rCBV values than native lesions, earlier diagnosis of recurrent tumor has been suggested using perfusion imaging than with serial MR imaging or with nuclear imaging techniques. In a series of 59 patients, perfusion imaging (rCBV maps) was found to predict tumor progression earlier than MR imaging in 32%, earlier than 201Tl-SPECT in 63%, and earlier than clinical assessment in 55% of the studies.81 Sugahara et al. evaluated 20 patients with the diagnosis of recurrent tumor versus radionecrosis by a combination of perfusion imaging and thallium singlephoton emission tomography (201Tl-SPECT). The rCBV values overlapped between neoplastic and nonneoplastic lesions, and the authors suggested that 201Tl-SPECT may be useful in making the differentiation.82 In view of an improved spatial resolution in comparison to SPECT and PET,83 perfusion MRI is a promising technique that needs to be evaluated further in clinical studies.

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