Plenary Oral Presentations - Macquarie University Hospital

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16 th International Meeting of the Leksell Gamma Knife ® SocietyMarch 2012, Sydney, AustraliaPH-102Clinical Implementation of the Gamma Knife Extend System:A Therapist’s Perspective2Amanda Schoenberger, 3 Robert Dryzmala, 3 Joseph Simpson, 2 Eric Filiput, 2 Christine Bossung,2Karen Watts, 2 Bridget Burke, 2 Barb Kienstra, 2 John Reed1Gamma Knife Center of Saint Louis2Barnes-Jewish Hospital3Washington University Medical SchoolThe Extend System, a non-invasive stereotactic Gamma Knife Perfexion accessory, permits fractionatedand sub-cranial treatments. The technique is useful when treating larger targets in previouslytreated regions or those close to adjacent risk zones. We report on our initial experience with tenpatients throughout bite-block fabrication, imaging, planning and treatment. We consider the impactof body habitus and location of the target on patient selection.Our initial experience with this technology has taught us valuable lessons for using the Extend System.We have developed procedural tips to aide in its success. Appropriate fabrication of positioning aidsand a comfortable patient are keys to reproducible patient set-up and treatment. Bite-block fabricationincludes a dental impression that is formed to the patient’s mouth. A plug is appropriatelydepressed into the impression material establishing the necessary opening for firm suction to the hardpallet. A vacuum pillow molds to the posterior head and neck. A mark placed at the top of the moldhelps to insert the patient’s head properly for each treatment. A measurement from the arch of thebite-block assembly to the patent’s anterior neck or chest records the height for future setting of thetreatment couch and CT couch during stereotactic imaging. Suction is then applied requiring propertechnique in order to maintain the suction level necessary for patient immobilization. Once the requiredsuction level is reached, the repositioning tool is used to record depth measurements on four sidesof the skull for the CT simulation process. These values provide guidance for duplication of set-up foreach subsequent treatment.Similar to treatment planning for the G-frame, a team of certified Gamma Knife users are required.The site, dose distribution and fractionation pattern are established by this team. Those patientsappropriate for the Extend System are usually given treatments of 3 to 5 fractions over the courseof 2 weeks.This innovative technology has proven successful for multiple patients at our institution in the hypofractionatedtreatment of gliobastoma multiforme, meningioma, leiomyosarcoma, skull base metastases,and brain stem lesions.56
16 th International Meeting of the Leksell Gamma Knife ® SocietyMarch 2012, Sydney, AustraliaPH -106Application of gamma evaluation method inGamma Knife film dosimetry3Hyun-Tai Chung, 1,2 Jeong-Hoon Park, 1 Jung Ho Han, 1,3 Chae-Yong Kim, 1,3 Chang Wan Oh,4Dohee Lee, 2 Tae-Suk Suh, 3 Dong Gyu Kim1Department of Neurosurgery, Seoul National University Bundang Hospital, Korea2Department of Biomedical Engineering, Catholic University of Korea, Seoul, Korea3Department of Neurosurgery, Seoul National University College of Medicine, Korea4Department of Neurosurgery, Asan Medical Center, Seoul, KoreaObjective: Gamma Knife (GK) radiosurgery is a minimally invasive surgical technique for the treatmentof intracranial lesions. It requires sub-millimeter accuracy to minimize neurologic deficits. In thispaper, the delivery accuracy of GK radiosurgery was assessed using gamma evaluation method forplanning dose distribution and film measurement data.Methods: With GK Perfexion (PFX) treatment planning system (TPS), single 4mm, 8mm, 16mm andcomposite shot plans were developed for evaluation. Their planning dose distributions were exportedas DICOM RT files using new function of GK TPS. Maximum dose of 8 Gy was prescribed for fourtest plans. They were irradiated to spherical solid water phantom with GafChromic EBT2 films inaxial and coronal plane. The exposed films were converted to absolute dose by 4th-order polynomialcalibration curve determined with ten calibration films. The film measurement results and planningdose distributions were registered in same Leksell coordinate using in-house software for furtheranalysis. The gamma evaluation method was applied to two dose distributions with various spatialtolerance of 0.3~2.0mm and dosimetric tolerance of 0.3~2.0% to verify the accuracy of GK radiosurgeryinversely. The result of gamma evaluation was assessed with pass rate, dose gamma indexhistogram (DGH) and dose pass rate histogram (DPH).Results: The 20, 50, 80% isodose lines found in film measurement showed close agreement withplanning isodose lines for all dose levels. The comparison of diagonal line profiles across axial planegave similar results. The gamma evaluation method resulted in high pass rates more than 95% within50% isodose line for 0.5mm/0.5% tolerance criteria in both axial and coronal planes. They satisfied1.0mm/1.0% criteria within 20% isodose line. Our DGH and DPH also showed low isodose lineshave inferior gamma indexes and pass rates than higher ones.Conclusions: It was possible to apply gamma evaluation method to GK radiosurgery. For all testplans, 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.57
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