Application of Gamma Evaluation Method in Gamma Knife Film Dosimetry

Hyun-Tai Chung, PhD1, Jung Ho Han2, Chae-Yong Kim3, Chang Wan Oh3, Dohee Lee4, Tae-Suk Suh5, Dong Gyu Kim6

1Seoul National University College of Medicine 2Gyeonggi-Do, Korea, Republic Of 3Department of Neurosurgery, Seoul National University Bundang Hospital, Korea 4Department of Neurosurgery, Asan Medical Center, Seoul, Korea 5Department of Biomedical Engineering, Catholic University of Korea, Seoul, Korea 6Department of Neurosurgery, Seoul National University College of Medicine, Korea

Keywords: physics, Dose distribution, gamma knife, dosimetry, radiosurgery

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     Gamma Knife (GK) radiosurgery is a minimally invasive surgical technique for the treatment of intracranial lesions. It requires sub-millimeter accuracy to minimize neurologic deficits.
     In this paper, the delivery accuracy of GK radiosurgery was assessed using gamma evaluation method for planning dose distribution and film measurement data.
     With GK Perfexion (PFX) treatment planning system (TPS), single 4mm, 8mm, 16mm and composite shot plans were developed for evaluation. Their planning dose distributions were exported as DICOM RT files using new function of GK TPS. Maximum dose of 8 Gy was prescribed for four test plans. They were irradiated to spherical solid water phantom with GafChromic EBT2 films in axial and coronal plane. The exposed films were converted to absolute dose by 4th-order polynomial calibration curve determined with ten calibration films. The film measurement results and planning dose distributions were registered in same Leksell coordinate using in-house software for further analysis. The gamma evaluation method was applied to two dose distributions with various spatial tolerance of 0.3~2.0mm and dosimetric tolerance of 0.3~2.0% to verify the accuracy of GK radiosurgery inversely. The result of gamma evaluation was assessed with pass rate, dose gamma index histogram (DGH) and dose pass rate histogram (DPH).
     The 20, 50, 80% isodose lines found in film measurement showed close agreement with planning isodose lines for all dose levels. The comparison of diagonal line profiles across axial plane gave similar results. The gamma evaluation method resulted in high pass rates more than 95% within 50% isodose line for 0.5mm/0.5% tolerance criteria in both axial and coronal planes. They satisfied 1.0mm/1.0% criteria within 20% isodose line. Our DGH and DPH also showed low isodose lines have inferior gamma indexes and pass rates than higher ones.
     It was possible to apply gamma evaluation method to GK radiosurgery.
     For all test plans, planning dose distribution and film measurement met the tolerance criteria of 0.5mm/0.5% within 50% isodose line being used for marginal dose prescription.


Project Roles:

H. Chung (), J. Han (), C. Kim (), C. Oh (), D. Lee (), T. Suh (), D. Kim ()