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Comparison of CT and MRI Based Monte Carlo Simulation for Gamma Knife Stereotactic Radiosurgery

W Xiong

W Xiong1*, D Huang1, (1) Memorial Sloan-Kettering Cancer Center, Rockville Center, NY

SU-E-T-540 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:MRI image is most frequently used for target contouring and treatment planning in Gamma Knife stereotactic radiosurgery (SRS). This study is to compare geometric and dosimetric accuracy of CT and MRI-based Monte Carlo (M.C.) simulation for Gamma Knife SRS.

Methods:A cylindrical water phantom with scale for MRI QA was scanned and the MRI images were transferred to a planning system for geometric analysis. M.C. simulation was applied on patient geometries reconstructed from CT and MRI data for dosimetric comparison. In the M.C. simulation, Gamma Knife (Model C) unit geometry and material were reconstructed according to original unit. A heterogeneous patient MRI geometry was created by putting a 1.8 g/cc skull in the unity homogeneous MRI geometry based on MRI anatomy knowledge. The dose was calculated using M.C. simulation in both homogenous and inhomogeneous CT and MRI geometries with identical beam parameters. The dose distribution was compared by overlapping the isodose-lines for each calculation. The DVH was derived by collecting dose on a small volume around isocenter.

Results:In MR image, the maximum errors along all directions are within 0.5 mm in the volume of interest (VOI) which is about 15cm high and 20cm diameter in x and y plane.
There is no observable difference of relative isodose lines in CT and MRI geometries. However, the absolute dose in heterogeneous CT geometry was 3.2% lower than the dose in homogeneous CT geometry from the DVH comparison. The absolute dose in homogeneous MRI phantom was 3.3% higher that dose in heterogeneous CT geometry. After applying heterogeneity correction to the skull for MRI, the difference was reduced to less than 2%.

Conclusion:MRI image distortion is small with the maximum distortion within 0.5mm in VOI. MRI-based Monte Carlo planning for Gamma Knife is feasible after applying proper skull heterogeneity correction.

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