Plenary Oral Presentations - Macquarie University Hospital

16 th International Meeting of the Leksell Gamma Knife ® SocietyMarch 2012, Sydney, AustraliaPH-296Monte Carlo calculated and experimentally verifiedcorrection factors for clinical reference dosimetry forthe Leksell Gamma Knife ® : Application of a new IAEAdosimetry formalism1Jonas Johansson, 1 B. Górka, 2,4 J. Novotny Jr, 3 J.D. Bourland, 2 J. Bhatnagar, 2 G. Bednarz,3R.C.M. Best, 5 L. Persson, 1 H. Nordström, 1 O. Svärm1Elekta Instrument AB, Box 7593, SE-103 93, Stockholm, Sweden2University of Pittsburgh Cancer Institute, Pittsburgh, PA 15213, USA3Wake Forest School of Medicine, Winston-Salem, North Carolina 27157, USA4Na Homolce Hospital, Prague 150 30, Czech Republic5Swedish Radiation Safety Authority, Solna strandväg 96, 171 16 StockholmObjective: Dosimetrical measurements for the Leksell Gamma Knife® (LGK) are challenging due tothe possible lack of electronic equilibrium, spectral changes with field size, and partial occlusion ofthe primary source. The IAEA working group on reference dosimetry of small and nonstandard fieldshave published a new dosimetry formalism. The objective of this work is to report Monte Carlo (MC)calculated and experimentally verified correction factors for a number of ionization chambers that areneeded for the application of the new IAEA formalism to LGK dosimetry.Methods: According to the new IAEA formalism, the absorbed dose to water is given byThis formalism links the standard reference field (f ref) used at a SSDL and the machine specific referencefield (f msr), used in the clinic, via the correction factor . Correction factors for eight differentsmall volume ionization chamber types (0.007-0.125 cm3) from various manufacturers used for LGKdosimetry were determined using MC simulations. Verification of MC simulated correction factorswere done at three LGK facilities and the Swedish SSDL through an international collaboration.Measurements followed a prescribed procedure to provide consistency and accuracy in the acquireddatasets. The absorbed dose to water was determined using IAEA TRS-398 and the new IAEA smallfield formalism.Results: Applying the correction factor using the new formalism leads to, depending on the ionizationchamber and phantom configuration, an increase in the dose-rate between 0.8%-1.6% for PFX and0.3%-0.5% for LGKC compared to IAEA TRS-398. The average standard deviation in the dose-ratemeasurement decreases from about 1% applying TRS-398 to about 0.6% applying the new formalism.Conclusion: With the new formalism, average dose rate is higher and the standard deviation isdecreased slightly compared to IAEA TRS-398. This suggests that small improvements in the accuracyof Gamma Knife ® dosimetry can be accomplished through the implementation of the new formalismusing the correction factors reported in this study.69