Post SRS Serial Cellular Density Mapping for Response Assessment in Vestibular Schwannoma

Abstract

Diffusion-weighted (DW) imaging detects cellular changes. Loss of tumor cells post radiosurgery results in alteration in the Cellular density and thus changes in apparent diffusion coefficients (ADCs). Low ADC values represent a higher tumor cellularity, a higher tumor-grade and may even be a marker for a higher tumor activity. Purpose of the study is to investigate the clinical usefulness of ADC values for evaluating VSs and to assess the correlation of DW imaging measurements with radiosurgical effects on the tumors.The authors conducted a retrospective evaluation involving 36 patients harboring solid (28 pts.) or cystic vestibular schwannomas (8 pts.) who underwent single fraction Linac based SRS. The patients underwent serial MR imaging, including DW imaging, before SRS and at 6mo. intervals thereupon following the procedure. The ADCs were calculated from echo-planar DW images, and mean ADC values were compared at each follow-up. To minimize the effect of diffusion anisotropy, an average of the values obtained for the 3 diffusion directions was calculated to provide the trace of the diffusion tensor. Data were analyzed using a t-test to compare the mean ADC values obtained before & after SRS.The mean follow-up period in this study was 32.5 months (range 16 -56 months). Imaging studies demonstrated a reduction in tumor volume in 22 patients (61.3%) and tumor growth arrest in 11 patients (33%). There was tumor enlargement at 18, 36, and 42 months in the remaining 3 patients (8.3%). The mean ADC value before SRS for all solid VSs was 1.2 ± 0.17 × 10-3 mm2/second, which significantly increased 6 months after SRS and continued to increase with time (P = 0.0076). The mean ADC value for treated solid tumors as of the last mean follow-up of 36 months (range 18-60 months) was 1.92 ± 0.26 × 10-3 mm2/second (range 1.60-2.7 × 10-3 mm2/second), which was significantly higher than that before SRS (P = 0.0001). Tumor volumes were positively related to ADC values (P = 0.03). The mean ADC value before SRS for all cystic VSs was 2.09 ± 0.24 × 10-3 mm2/second (range 1.80-2.58 × 10-3 mm2/second). The mean ADC value for treated cystic tumors as of the last mean follow-up of 38 months (range 18-48 months) was 1.9 ± 0.42 × 10-3 mm2/second. In 3 patients harboring solid VSs, the tumor enlarged after SRS but the ADC values were higher than those before SRS.ADC changes precede the real change in tumor size detected on routine follow-up MR images. ADC values of solid tumors significantly increased, even before a change in tumor volume was evident and should be incorporated in VS routine MR imaging post SRS.V. Shankar: None. V. Sai Shreya: None. R. Kumar: None. C. Haritha: None. A.K. Karthikayan: None. R. Adhityan: None. P. Bhavya: None. H. Vyas: None. A. Bhange: None.