Pretreatment ADC Values Predict Response to Radiosurgery in Vestibular Schwannomas

BACKGROUND AND PURPOSE: The response rate of vestibular schwannomas to radiation therapy is variable, and there are surgical options available in the event of treatment failure. The aim of this study was to determine whether pre- and posttreatment ADC values can predict the tumor response to radiation therapy. MATERIALS AND METHODS: From a data base of 162 patients with vestibular schwannomas who underwent radiation therapy with gamma knife, CyberKnife, or fractionated stereotactic radiation therapy as the first-line therapy between January 2003 and December 2013, we found 20 patients who had pretreatment ADC values. There were 108 patients (including these 20) had serial MR images that included DWI allowing calculated ADC values from 2-132 months after radiation therapy. Two reviewers measured the mean, minimum, and maximum ADC values from elliptical ROIs that included tumor tissue only. Treatment responders were defined as those with a tumor total volume shrinkage of 20% or more after radiation therapy. RESULTS: The pretreatment mean minimum ADC for nonresponders was 986.7 x 10(-6) mm(2)/s (range, 844-1230 x 10(-6) mm(2)/s) and it was 669.2 x 10(-6) mm(2)/s (range, 345-883 x 10(-6) mm(2)/s) for responders. This difference was statistically significant (P < .001). Using a minimum ADC value of 800 x 10(-6) mm(2)/s led to the correct classification of 18/20 patients based on pretreatment ADC values. The intraclass correlation between reviewers was 0.61. No posttreatment ADC values predicted response. CONCLUSIONS: Pretreatment ADC values of vestibular schwannomas are lower in responders than nonresponders. Using a minimum ADC value of 800 x 10(-6) mm(2)/s correctly classified 90% of cases.