Radiation injury is one of the most important problems after stereotactic radiosurgery (SRS) and radiotherapy (SRT). However differential diagnosis between radiation injury and tumor recurrence is difficult.

     We evaluate availability of 11C-Methionine positron emission tomography (Met-PET) in differential diagnosis after SRS and SRT.

     Between January 2008 and May 2010, 23 patients who had expanding lesion of brain metastasis after stereotactic radiosurgery and radiotherapy were engaged on this study.

     Between January 2008 and May 2010, 23 patients who had expanding lesion of brain metastasis after stereotactic radiosurgery and radiotherapy were engaged on this study. Met-PET images were obtained as a static scans of 10 minutes performed 20 minutes after injection of 11C-methionine. On Met-PET scans, the portion of the tumor with the highest accumulation was selected as the region of interest (ROI), tumor-versus-normal ratio (TN ratio) was defined as the ratio of the standardized uptake value (SUV) in the tumor (T), divided by SUV in normal gray matter (N). Met-PET scan accuracy was evaluated by histological assessment from removed tissue (8 cases) or, in cases without surgery or biopsy, by the subsequent MR findings (15 cases).

     Maximum TN ratio was 1.28 in the radiation necrosis (15 cases) and 1.78 in the tumor recurrence (8 cases). There were statistically significant differences between tumor recurrence and radiation injury in SUV. The cut-off value of SUV is about 1.5. Subsequent Met-PET was performed in 5 patients who had around 1.5 in the SUV. All of these cases could arrive at differential diagnosis.

     This was a retrospective study.

     Met-PET proved useful for differential diagnosis for radiation injury and tumor recurrence.

     If the initial Met-PET could not reach diagnosis, subsequent examination was available for differential diagnosis.