Plenary Oral Presentations - Macquarie University Hospital

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16 th International Meeting of the Leksell Gamma Knife ® SocietyMarch 2012, Sydney, AustraliaPH-200International Leksell Gamma Knife Calibration Survey– Final Results1,3Josef Novotny Jr., 2 Marc Desrosiers, 1 Jagdish Bhatnagar, 3 Josef Novotny, 4 Masaaki Yamamoto,1Saiful Huq, 2 James Puhl, 5 Dade Lunsford1Department of Radiation Oncology, University of Pittsburgh Cancer Institute, Pit2National Institute of Standards and Technology, US Department of Commerce, Gaith3Department of Medical Physics, Na Homolce Hospital, Prague, Czech Republic4Katsuta Hospital Mito Gamma House, Hitachinaka, Japan5Department of Neurological Surgery, University of Pittsburgh Medical Center, PitObjective: Survey approximately 100 Leksell Gamma Knife (LGK) units worldwide to 1) gather detailedinformation about calibration procedures and to 2) measure output of the surveyed LGK units.Methods: Each participant of the project received a LGK calibration questionnaire seeking thefollowing information: LGK model, calibration protocol, phantom, and ion chamber used for calibration,LGK calibration personnel, whether independent verification of calibration was performed, andrelative collimator output factors used. Alanine dosimeters were used to measure the dose rate of eachsurveyed LGK unit and these results compared with calibration data.Results: To date, 70 LGK units from 66 different centers spanning 15 different countries haveparticipated in this study (35 North America, 13 Europe, 21 Asia and 1 South America). Thecalibration protocols used are: AAPM TG21 (26), AAPM TG51 (7), IAEA TRS277 (1), IAEATRS398 (29), NPL Code of Practice (5) and DIN 6800-2 (2). ELEKTA ABS phantom was used in 65(93%) and ELEKTA solid water phantom in 5 (7%) units. Ion chambers most frequently used forthe calibration are: PTW 31010 (0.125 cm3) 26 times (37%), Exradin A16 (0.007 cm3) 12 times(17%) and Capintec PR-05P (0.070 cm3) 11 times (16%). Calibration of the LGK was performed byan on-site physicist in 52 (75%) cases, by ELEKTA physicist in 17 (24%) cases and in 1 (1%) casecalibration was done by a consulting physicist. Independent verification was done only in 31 (44%)cases; RPC, IAEA or similar audit was done only in 20 (29%) cases. All LGK units surveyed arecurrently using the default values for collimator relative output factors. Observed deviations betweenLGK users reported calibration and alanine dosimetry measurements were of small magnitude withmean value of 1.6%. In total 63 (90%) LGK units were within 3% deviation. None of measured unitsexceeded 4% deviation in calibration.Conclusions: Different calibration procedures, especially calibration protocols are used worldwide.Small but systematic deviation between LGK users reported calibration and alanine dosimetrymeasurements are observed for LGK centers in Europe and Asia where IAEA TRS398 protocolis used. This can be explained by the fact that the ABS plastic phantom is considered to be waterequivalent.62
16 th International Meeting of the Leksell Gamma Knife ® SocietyMarch 2012, Sydney, AustraliaPH-236Improvement of the total clinical accuracy ofgamma knife radiosurgery and assessment of theclinical impact of MR spatial distortions2Michael Torrens, 1,2 Pantelis Karaisko, 1 Argyris Moutsatos, 3 Ioannis Seimenis,1Panagiotis Papagiannis, 1 Evangelos Georgiou, 2 Christos Stergiou1Medical Physics Department, Medical School, University of Athens, Greece2Gamma Knife Department, Hygeia Hospital, Athens, Greece3Medical Physics Department, Medical School, University of Thrace, GreeceObjective: The total clinical accuracy of GK treatment delivery may be impaired due to the spatialdistortion inherent in the MR images used for target volume definition in 3D space. This work presentsa time-efficient method to improve the total clinical accuracy of GK radiosurgery by correcting theMR images for background field distortions which, as we have published previously, are a majorcontributor to accuracy degradation (~1mm).Methods: The proposed method is based on the acquisition of an extra MR sequence that adds onlyminutes to the total patient scan-time and an appropriate software tool written in Matlab to processthe “corrected” MR series. The method was benchmarked at 1.5 T using phantoms and then usedto assess the effect of the sequence–dependent spatial distortions in 12 patient treatments with a totalnumber of 80 relatively small metastases (less than 2cm in diameter, with 88 % of them less than1cm and 55% less than 0.5cm). Using the extra MR sequence acquired for all patients, a “corrected”MR series was produced in one-to-one correspondence for every “original” Gd-enhanced T1W seriesroutinely used for target definition. Then the targets corresponding to the different metastases werecontoured on both the “original” and “corrected” series. Patient plans based on the “original” MRseries were finally produced.Results: The prescription dose was 25 Gy and the target coverage more than 99% for all targetscontoured in the “original” MR images. For the same plans, the 25Gy target coverage was found tobe less than 93% for 41% of the targets contoured in the “corrected” MR images (54% of the targetswith diameter 1cm). For thesetargets, a dose of less than 23Gy (18–23Gy) instead of the prescribed 25Gy covered the 99% of thetarget volume (dose difference >8%).Conclusions: There is a fundamental inaccuracy induced by background field spatial distortions onMRI. Small targets especially may be incorrectly covered by routine planning methods. The proposedmethodology was found to minimize the distortions and thus to improve significantly (in sub-voxeldimensions) the total clinical accuracy.63
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