Dose verification approach applicable to Leksell Gamma Knife Radiosurgery: Variable Ellipsoid Modeling Technique(VEMT)

Beong Ik Hur1, Dong Wan Kang2, Won Ho Cho3, Seung Heon Cha3, Chang Hwa Choi3, Byung Kwan Choi3

1 2Department of Neurosurgery, Pusan National University 3Department of Neurosurgery, Pusan National University School of Medicine

Keywords: physics, dosimetry, dose planning, gamma knife, radiosurgery

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     The independent verification of Leksell GammaPlan(LGP) is extremely important in order to assure patient’s safety and minimize the risk of treatment errors.  
     We assessed the calculated results of LGP for the randomly selected 10 patients with metastasis brain tumors treated using Leksell Gamma Knife-based stereotactic radiosurgery.
     The main problem of the treatment planning system is one of how to model the geometry of a skull and how to subsequently calculate numerous attenuation depths through the modeled skull. In our algorithm, we simulated the skull shape as an ellipsoid with its center at the junction between the mammillary bodies and the brain stem. The radiation depths of the beamlets emitted from 201 collimators (Gamma Knife type B, C and 4C are the same geometrical structure.) were calculated based on the relationship between this ellipsoid and a single beamlet expressed as a straight line. Our dose verification algorithm ignores the scattered photon contribution and assumes homogeneous patient geometry like LGP. And the 201 Cobalt-60 sources are assumed to have the same radioactivity. A computer program was coded to execute the algorithm. A database system was adopted to log in the dose for 31 X 31 X 31 or 29,791 lattice points to query the matrix of interest. Our approach can be easily applied to multiple matrixes.
     When we compared the physically, medically interesting parameters in 10 cases with metastasis tumors treated at our center, main parameters -maximal dose location, maximum dose, dose at an arbitrary location, and tumor volume- showed good correlation between VEMT and LGP. The discrepancy of dose at an arbitrary location showed the good agreements with average -0.5%. Also the maximal dose location and maximum dose were obtained with average difference 1.6mm, 0.86%. Each of them showed the good results. But we had the different results in two cases of the comparison of the target volume.
     Variable Ellipsoid Modeling Technique(VEMT) we developed is a useful, independent tool to verify the important parameters of Leksell GammaPlan(LGP) as a part of quality assurance.
     Independent verfication tools can be of value.


Project Roles:

B. Hur (), D. Kang (), W. Cho (), S. Cha (), C. Choi (), B. Choi ()