Object: Gamma Knife surgery (GKS) is often the sole treatment for brain metastases. The authors hypothesized that early post-GKS measures of the relative apparent diffusion coefficient (rADC) could predict therapeutic response, recurrence, and radionecrosis prior to changes in tumor volume.
Methods: Magnetic resonance (MR) images of 25 metastatic tumors in 15 patients were reviewed. Inclusion criteria included a history of surgery or GKS, a minimum tumor diameter of 5 mm at treatment, and a minimum of two follow-up MR images. Tumor volumes were normalized to baseline, and tumor ADC values were normalized to normal-appearing white matter (rADC). A successful therapeutic response (STR) was defined by a monotonically decreasing tumor volume throughout the follow-up period. Magnetic resonance spectroscopy was used to classify non-STRs as radionecrosis or tumor recurrence. All tumors exhibited a decreased normalized volume (mean 37%) at the first follow-up examination (range 33-124 days after GKS, mean 54 days), and three distinct rADC patterns subsequently evolved: Group 1 (STR [10 cases]), monotonically decreasing volume with gradually increasing rADC; Group 2 (radionecrosis [three cases]), initial volume reduction followed by gradual increase, with initial rapidly increasing rADC followed by more gradual increase or plateau; and Group 3 (recurrent tumor [12 cases]), initial volume decrease followed by increase, with a preceding gradual decrease in the rADC.
Conclusions: The rADC patterns outperform initial post-GKS tumor volume in predicting the long-term response to treatment. Decreasing tumor volume with an increasing rADC predicts an STR. For lesions with increasing volume, antecedent rADC reduction predicts recurrence, whereas a rapidly increasing rADC predicts radionecrosis. Evaluation of the rADC at the initial post-GKS follow-up examination appears to be a useful prognostic measure of metastatic tumor response.