Pretreatment ADC Values to Predict Response of Vestibular Schwannoma to Gamma Knife Radiosurgery

Background: Pretreatment tumor ADC values have been shown to be useful in predicting response of glioblastomas and vestibular schwannomas treated with radiation therapy. Particularly, a prior study highlighted the minimum ADC of a pretreated vestibular schwannoma in predicting response. The purpose of the study is to validate the use of pretreatment ADC values in vestibular schwannomas as a predictor of response to gamma knife radiation therapy with a larger study population.

Materials and Methods: Search of an imaging database for patients with known vestibular schwannoma who received gamma knife radiosurgery at our institution was conducted. Included patients had a pretreatment MR allowing for measurement of tumor volume, adequate determination of an ADC value, and a follow-up MR of greater than 12 months from treatment, yielding 48 patients who were treated between June 2008 and August 2016. Three reviewers measured minimum, mean and maximum pretreatment ADC values from ROIs proportionate to tumor size. The pretreatment ADC values were standardized to white matter to compensate for differences in acquisition parameters and scanners in which MR examinations were performed. Pre- and posttreatment tumor volumes were calculated through the summed areas of multiple cross-sectional images through the tumor, and treatment response was defined as a decrease of 20% or more in total volume. Follow-up MR examinations with mean time from treatment of 12 to 102 months were used to measure total tumor volume and determine change in size following treatment.

Results: Average age of the 48 patients included in the study was 63 years, with 20 males and 28 females. Average pretreatment tumor volume was 1.92 cm³, and the average length of follow-up was 39 months. A multi-variable linear regression analysis was performed to predict the percent tumor change due to Gamma Knife treatment based on the standardized ADC minimum, ADC mean, ADC maximum, length of follow-up and pretreatment tumor size. No significant regression equation was found between the variables and percent tumor size change (F (5, 42) = 1.072, p = 0.390, R ² of 0.008). Individual variable predicted percent change was also not significant when correlated with ADCmin (B = −0.423, p = 0.351), ADCmean (B = 0.545, p = 0.563), ADCmax (B = −0.206, p = 0.694), follow-up months (B = −0.671, p = 0.094), and pretreatment volume (B = −4.215, p = 0.413).

Additionally, when defining responders to treatment as 20% or more decrease in volume after gamma knife with the rest as nonresponders similar to a recent published article, an independent samples t-test was performed that showed no difference between the standardized ADC minimum (p = 0.369), ADC mean (p = 0.945), ADC maximum (p = 0.627), and pretreatment tumor size (p = 0.917) to differentiate between responders and nonresponders. The number of months of follow-up after treatment did significantly differentiate between responders and nonresponders (p = 0.004), with responders having average follow-up of 44.2 months and nonresponders having average follow-up of 26.8 months.

Conclusion: This study demonstrates that lower pretreatment ADC values did not predict reduction in vestibular schwannoma size after gamma knife therapy. This study with a larger patient population is in contradistinction to previously reported findings.