MR spectroscopy in the evaluation of recurrent contrast-enhancing lesions in the posterior fossa after tumor treatment.

Recurrent contrast-enhancing lesions arising within foci of prior brain neoplasms treated with chemotherapy and/or radiation therapy pose a significant diagnostic dilemma, as they may represent recurrent or residual tumor, treatment-related changes, or a combination of both. Those lesions specifically in the posterior fossa are even more difficult to assess, given the technical limitations of 2D CSI in the infratentorial compartment. We explored the feasibility of 2D-CSI MR spectroscopy in the evaluation of recurrent contrast-enhancing lesions in eight consecutive patients who had undergone treatment for posterior fossa or brainstem tumors. Mean Cho/Cr (choline/creatine) ratios obtained by 2D-CSI in recurrent tumor, treatment-related changes, and normal white matter were 2.93, 1.62, and 0.97, respectively, mean Cho/NAA (choline/N-Acetyl aspartate) ratios were 4.34, 1.74, and 0.93, and mean NAA/Cr (N-acetyl aspartate/creatine) ratios were 0.74, 0.92, and 1.26, respectively. In conclusion, also in the posterior fossa, MR spectroscopy is likely to be useful as an adjunct to conventional imaging characteristics in distinguishing recurrent tumor from treatment-related changes, irrespectively of the MRS technique used. In most cases spectra of diagnostic quality can be obtained using 2D-CSI to include coverage of both the lesion and its vicinity.