11C-Methionine (MET) is a useful positron emission tomography (PET) tracer for the evaluation of low-grade gliomas. Among these tumors, a high percentage of low-grade oligodendrogliomas (ODG) are sensitive to chemotherapy with procarbazine, CCNU, and vincristine (PCV). We aimed at: (1) objectively assessing ODG response to PCV by a metabolic index (the Activity Volume Index or AVI) generated from an automated semi-quantification of PET with MET (PET-MET); (2) comparing AVI and quantitative magnetic resonance imaging (MRI) measurements of response to PCV.
Methods: seven patients with ODG were followed for a period of 19.9+/-6.6 months after the completion of PCV chemotherapy. Regions of interest (ROI) were generated by covering all voxels with count values above a threshold level set at 120% of the mean cerebellar activity. On each slice, ROI volume and mean count values were calculated. AVI was calculated as the sum over all ROI of tumor volumex(tumor mean count/cerebellum count). Tumor volume measurements on MRI, were based on signal abnormalities visually detected on fluid-attenuated inversion recovery (FLAIR) sequences.
Results: PCV therapy was associated with a drastic decrease in AVI (mean+/-SD, cm3): AVI post-PCV=0.80+/-1.45 vs. AVI prior PCV=12.94+/-11.46 (P=0.03). Likewise, we observed a decrease in tumor volume estimated from the FLAIR signal (31.37+/-11.99 post-PCV vs. 67.95+/-39.96 prior PCV, P=0.03) although AVI decrease after PCV was significantly more pronounced (P=0.015).
Conclusion: This study, based on limited number of patients and follow-up period indicates that AVI may be a sensitive and observer-independent method applicable to the assessment of ODG responsiveness to PCV treatment and may offer a major added value to both clinical assessment and MRI evaluation of chemotherapeutic outcomes.