Application of diffusion-weighted magnetic resonance imaging to predict the intracranial metastatic tumor response to gamma knife radiosurgery

To evaluate the effect of stereotactic radiosurgery (SRS) on intracranial metastases with diffusion-weighted imaging/apparent diffusion coefficient maps. A total of 107 patients with 144 metastases larger than 1 cm in diameter were retrospectively reviewed. We calculated the DWI_{Tumor/white matter} ratios (DWI_T/WM ratio) between the metastases and the normal, contralateral frontal white matter at each time point. We also recorded the ADC values for metastases (ADC_T values). The DWI_T/WM ratio and ADC_T values were assessed for correlation with the patients' tumor response, brain edema, and survival. A decrease in DWI_T/WM ratios was seen in the controlled metastases, and an increase in the DWI_T/WM ratio were seen in the metastases with poor tumor control. On the other hand, an increase in ADC_T values was seen in the controlled metastases, and a decrease in ADC_T values was seen in the metastases with poor control. The differences were significant (p value: 0.001 and 0.002, respectively). Sensitivity of a decrease in the DWI_T/WM ratio to make an early prediction of tumor control was 83.9 %, and specificity was 88.5 %. When using the initial ADC_T values of metastases to predict tumor response, sensitivity and specificity were 85.5 and 72.7 %, respectively. DWI/ADC is a practical method for studying the efficacy of SRS and predicting early metastases response progression. A decrease signal on DWI and increased ADC values are indicators of good tumor control, and reflect the beneficial effect of SRS.