P16.22DIFFERENTIAL DIAGNOSIS OF TUMOR RECURRENCE AND RADIATION NECROSIS AFTER RADIOSURGERY FOR BRAIN METASTASES WITH 320-ROW AREA DETECTOR CT PERFUSION IMAGING

BACKGROUND AND PURPOSE: Radiation necrosis occurs from 6 months to several years following stereotactic radiosurgery (SRS) for brain metastases when tumor recurrences are also most likely. Conventional MR imaging does not provide sufficient information to differentiate between radiation necrosis and tumor recurrence. Tl-SPECT, FDG-PET, MET-PET and MR spectroscopy sometimes lead to false positive findings. We applied 320-row area detector CT perfusion imaging for the differentiation because it shows vascularity of lesions in the whole brain. METHODS: Between October 2008 and September 2013, 275 patients with metastatic brain tumors underwent SRS at our institute. Follow-up MR images were taken at 1 and 3 months and successive every three months after radiosurgery. Twenty-eight lesions enlarged in 26 patients 3 to 26 months (median: 8 months) after treatment. They underwent CT perfusion imaging. To calculate the normalized cerebral blood volume ratio (nCBV ratio), the regions of interest (ROIs) were located in the enhanced areas and in the contralateral normal white matter. Serial MR imaging with gadolinium was done at one- to two-month intervals afterward. Lesions progressively increased in size or became symptomatic were resected and diagnosed by pathological examination. On the other hands, lesions decreased or stable on MR images over three months were diagnosed as radiation necrosis. RESULTS: Surgical removal of the lesions revealed 16 of 18 patients had tumor recurrence. Serial MR imaging (10 patients) and surgery (2 patients) revealed 12 patients suffered radiation necrosis. CT perfusion images showed nCBV ratios higher than 3.0 in all 14 patients with recurrent metastases; whereas it demonstrated nCBV ratios lower than 3.0 in the 12 patients with radiation necrosis. An cut off value nCBV ratio of greater than 3.0 analyzed by ROC curve provided the best sensitivity and specificity for identifying recurrent metastatic tumors, at 81.3% and 100%, respectively. CONCLUSION: Perfusion CT imaging demonstrated a threshold of nCBV ratio reflecting microvascular information about the lesions, and is a reliable method for differentiating tumor recurrence from radiation necrosis after radiosurgery.