SU-E-T-536: Inhomogeneity Correction in Planning of Gamma Knife Treatments for Acoustic Schwannoma.
PURPOSE
To find out the dose difference on targets and organs at risk for the treatment of acoustic schwannoma if the inhomogeneity correction (Convolution algorithm) is applied.


METHODS
Images of patients treated for acoustic schwannoma with Gamma Knife using TMR 10 algorithm were retrieved from database and replanned with Convolution and TMR 10 algorithm respectively. These patients were treated using a preplan scheme in following: (1) Before the actual treatment day, using the MRI image that was taken without a head frame on the patient's skull, a pre-treatment plan was made based on the default skull coordinates in the Gamma Knife treatment planning system (LGP); (2) then on treatment day, a head frame was placed on the patient's skull, and a CT image was taken. The CT image with head frame was registered and fused with the completed preplan; (3) the treatment plan was finalized and the treatment was delivered. To find out the dosimetry impact of inhomogeneity correction, we used the retrieved CT images to replan the treatment using Convolution algorithm in LGP software version 10.1.1. The dose distributions and the dose volume histograms for targets and OARs were compared for these two dose calculation algorithms.


RESULTS
The dose calculated with the Convolution algorithm in general is slightly lower than the one from TMR 10 around the boney area. The effect from the inhomogeneity correction is observable but not significant, and varies with the location of the tumor.


CONCLUSION
Inhomogeneity correction slightly improve the dose accuracy for acoustic schwannoma Gamma Knife treatments although the correction may not be very significant. Our Result provides evidence for dose prescription adjustment to treat acoustic schwannoma. The actual clinical outcome of switching from using TMR10 to using Convolution needs to be further investigated.