Multimodal Approach using ¹¹C-methionine PET and special MR Techniques in Differential Diagnosis between Tumor Recurrence and Radiation Necrosis after Gamma Knife Radiosurgery for Metastatic Brain Tumors

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     When a tumor progression was detected after gamma knife radiosurgery (GKR) for metastatic brain tumors, it is important to distinguish tumor recurrence from radiation necrosis.
     We used multiple combined methods to diagnose differentially such two conditions, and the modalities in this study were 1)¹¹C-methionine Brain PET/CT, 2)MR spectroscopy, and 3) MR perfusion scan.
     The patients (n=20) had undergone GKR, and their tumors showed progress in volume during follow-up periods. The mean age was 67.2, and the primary cancers were nonsmall cell lung cancer (n=10), small cell lung cancer (n=5), colorectal cancers (n=3), and others (n=2). At GKR, the mean tumor volume and prescription dose were 8.0 cc and 17.8 Gy.
       The duration from GKR to the tumor progression ranged from 2 to 24 months (mean: 8.0). All three modalities were performed for the patients. In ¹¹C-methionine Brain PET/CT, we observed the ‘significant hot uptake of methionine’ (positive) in the tumors. If the choline/ creatinin (Cho/Cr ratio) was 1.5 or more in the tumors, we regarded it as ‘tumoral spectrum’ (positive). Increased perfusion to the tumor area compared to contralateral brain was regarded as ‘increased tumoral perfusion’ (positive).
     ¹¹C-methionine Brain PET/CT showed the ‘significant hot uptake of methionine’ in 16 patients, MR spectroscopy showed the ‘tumoral spectrum’ in 15 patients, and MR perfusion scan showed the ‘increased tumoral perfusion’ in 11 patients. The bases for ‘tumor recurrence’ were 1) histopathology of the tumor, 2) further volumetric progression in MRI, or 3)positive treatment effect of secondary GKR or radiation therapy. Four patients were compatible for the tumor recurrence, and all of them showed positive in the three parameters in this study. The bases for radiation necrosis were 1) further regression or 2) stable in size during meaningful duration. Three patients were compatible. Two of them showed positive in only one parameter (MRS and perfusion, respectively). The other patient showed positive in MRS and ¹¹C-methionine Brain PET/CT.
     This was a retrospective study.
     If we use above combined diagnostic modalities optimally, we could have an early chance to distinguish accurately the tumor recurrence from radiation necrosis after GKR for metastatic brain tumors.
     Validation of methods to optimally determine radiation effects from tumor progression are crucial.


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