The GammaPlan provides accurate estimation of target volume. However, a stereotactic frame and fiducials are required. At follow-up MR/CT, fiducials are no longer available. Surgeons rely on visual impression, 2-D measurements, or other methods to estimate the volume of the treated targets. These methods are not objective and may give rise to misleading conclusions. By modifying the image header files of GammaPlan version 3, it is possible to scale the images which are not acquired with stereotactic fiducials. The target is then mapped and its volume measured as usual. The target volumes in 7 patients were measured by the tweaked GammaPlan and compared with volume measurements using the standard version. The mean error was less than 2%. In a separate study, phantom syringes with known volumes of water were used for MR imaging. The tweaked version again gave accurate volume estimation of the phantom syringes with a less than 5% error for most cases. This method has subsequently been used in all Gamma Knife follow-up cases. Significant volume changes have been detected where conventional assessment showed no apparent change. Moreover, such volume changes correlated with clinical improvement or deterioration. It is recommended that all Gamma Knife users report tumor response by volume change. Our method is simple, reliable and does not require additional cost.

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