Use of Apparent Diffusion Coefficients in Evaluating the Response of Acoustic Neuromas to Gamma Knife Surgery

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     Cellular density is a major factor responsible for change in apparent diffusion coefficients (ADCs).
     We hypothesized that loss of tumor cells after gamma knife® surgery (GKS) might alter ADCs and utilized magnetic resonance imaging (MRI), including diffusion-weighted imaging (DWI), to detect cellular changes in brain tumors to evaluate the response to and efficacy of GKS.
     The mean follow-up period for the 31 patients was 36.5 months (range, 12–60 months).
     We conducted a prospective trial involving 31 patients with solid or cystic acoustic neuromas treated with GKS. The patients underwent serial MRI, including DWI, before GKS and at multiple intervals following GKS. We followed up the patients, evaluating the MRI findings and clinical outcomes, at 6-month intervals. We calculated ADCs from echo-planar DWI scans and compared the mean ADC values at each follow-up.
     Imaging studies showed a reduction in tumor volume in 19 patients (61.3%) and tumor growth arrest in 9 patients (29%). The mean ADC value for all the solid acoustic neuromas before GKS was 1.06 ± 0.17 ´ 10–3 mm2/s, which significantly increased 6 months after GKS and continued to increase with time (P = 0.0086). The mean ADC value for all the cystic acoustic neuromas before GKS was 2.09 ± 0.24 ´ 10–3 mm2/s (range, 1.80–2.58 ´ 10–3 mm2/s). The mean ADC value for tumors treated at the last mean follow-up of 36 months (range, 12–60 months) was 1.72 ± 0.26 ´ 10–3 mm2/s (range, 1.50–2.09 ´ 10–3 mm2/s), which was significantly increased as compared with that before GKS (P = 0.0001). Tumor volume was positively related to the ADC values (P = 0.03).
     This was a retrospective study.
     ADC values might be useful for evaluating treatment results before any definite volume change is detected in scans or for distinguishing radiation-induced necrosis from tumor recurrence in cases where other imagery is not definitive, as in patients with increased tumor volume or no volume change. 
     We suggest that ADC measurement be included in routine MRI examination for the evaluation of GKS results.


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