Plenary Oral Presentations - Macquarie University Hospital

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16 th International Meeting of the Leksell Gamma Knife ® SocietyMarch 2012, Sydney, AustraliaPH-291TMR 10: A comparison with TMR classic forclinical treatment plans1,2,3,4Ian Paddick, 5 Jonas Johansson1Cromwell Hospital Gamma Knife Centre,, London, UK2London Gamma Knife Centre, Barts Hospital, London, UK3Thornbury Radiosurgery Centre, Sheffield, UK4St. Elisabeth Gamma Knife, Tilburg, Netherlands5Elekta Instrument AB, Stockholm, SwedenObjective: Leksell GammaPlan 10.0 introduced an improved dose calculation algorithm, TMR 10,which entails new beam profiles, depth doses and output factors. This enhanced beam modelling hasmade inaccuracies of the previous algorithm, TMR Classic, apparent. Furthermore, multi-isocentricplanning techniques can amplify subtle differences in single shot profiles, which in turn can affectthe dose calculated, normalisation and beam on time. We aimed to quantify dosimetrical differencesbetween the two algorithms.Methods: 30 targets planned with TMR Classic for the Leksell Gamma Knife ® Perfexion (PFX)were retrospectively re-calculated using the TMR 10 algorithm. This method was repeated for 37targets planned for the Model B/C. The maximum dose, the mean dose to the target, and themaximum dose to the organs at risk (OARs) were compared.In a non-clinical investigation, the greatest dose discrepancy possible between the two algorithmswas also explored.Results: Assuming that TMR 10 correctly calculates dose to the skull, TMR Classic generallyunderestimated maximum Perfexion doses by 2.0% (range: -3.3% to 6.7%), mean doses 1.5%(range: -2.0% to 5.3%) and OARs 3.6% (range: -3.9 to 7.1%).Model B/C data differed by less. TMR Classic underestimated maximum doses by an average of 1.2%(range: -0.7% to 4.2%), mean doses 0.9% (range: -1.3% to 2.7%) and overestimated OARs by 1.8%(range: -0.9% to 4.5%).In a non-clinical setting, the largest discrepancy was for a plan with 8mm and 16mm shots withidentical x and y coordinates, but with the 8mm shot placed 13.0mm in the superior direction. TMRClassic recorded a maximum dose 18.1% lower than that of TMR 10.Conclusion: Dosimetrical differences between the two algorithms can be significant in certain circumstances.Fortunately, dose differences between TMR Classic for Model B/C (where most of our empiricaldata lies) and TMR 10 is minimal. Doses to OARs were typically underestimated by TMR Classic forPFX, but overestimated for the Model B/C. As a result, some users may be unintentionally giving dosesto OARs that are several percent higher with PFX. When quoting doses to critical structures with clinicalresults, users should recalculate the doses delivered using TMR 10.68
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 strandvÃ¤g 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
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Plenary Oral Presentations - Macquarie University Hospital

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