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254 CORRECTION FACTORS FOR IONIZATION CHAMBER DOSIMETRY IN TOMOTHERAPY AND CYBERKNIFE
S131 254 CORRECTION FACTORS FOR IONIZATION CHAMBER DOSIMETRY IN TOMOTHERAPY AND CYBERKNIFE A. Gago-Arias1, J. Pardo-Montero1, R. RodriguezRomero2, P. Sanchez-Rubio2, D.M. Gonzalez-Castano1, H. Palmans3, P. Sharpe3, F. Fayos4, E. Antolin4, R. Simon4, F. Gomez1 1 University of Santiago de Compostela 2 Servicio Radiofùsica. Hospital Universitario Puerta de Hierro 3 National Physical Laboratory 4 Servicio Radiofùsica Hospital Ruber Internacional Introduction: International protocols for clinical reference dosimetry of high energy photons recommend the determination of absorbed dose to water using an ionization chamber calibrated in terms of absorbed dose to water in a reference beam quality Q0 under reference conditions, usually defined as 10 cm x 10 cm square field and 100 cm source to axis or source to surface distance. However, some modern radiotherapy techniques, as e.g. TomoTherapy Hi-Art (ThomoTherapy Inc., Madison, WI) and Cyberknife Robotic Radiosurgery system (Accu- ray Inc., Sunnyvale, CA), cannot fullfil such conditions, and rely on a combination of a large number of small fields which has substantially increased clinical dosimetry uncertainties. This situation prompted the creation of an international working group on reference dosimetry of small and non standard beams through the cooperation of the International Atomic Energy Agency and the American Association of Physicists in Medicine. A new reference dosimetry protocol was proposed [R. Alfonso et al, Med. Phys. 35, 5179-5186 (2008)] to complement and extend the recommendations contained in existing protocols with the calculation of specific correction factors for non-standard fields. This proposal addresses static field dosimetry through the definition of machine-specific-reference field (msr), as close as possible to conventional reference conditions whenever they cannot be established by the machine. Composite field dosimetry is also studied by means of the so called plan class specific reference fields (pcsr), similar to clinical treatments, which include unit specific delivery features. This work presents an experimental evaluation of correction factors associated to ion- ization chamber reference dosimetry of non-standard fields in TomoTherapy Hi-Art and CyberKnife units. Correction factors were obtained by comparing ionization chamber mea- surements with alanine dosimetry traceable to primary standards. Both msr and pcsr fields were investigated, and the applicability of the new formalism to clinical treatments was additionally illustrated with clinical examples. Methods and Materials: Regarding the TomoTherapy unit investigation, the Exradin A1SL ionization chamber (Standard Imaging, Middleton, WI), supplied by the vendor with the TomoTherapy unit, was studied. A 10 cm x 5 cm field defined at 85 cm sourcedetector-distance, SDD, was chosen as msr field for measurement at 5 cm depth in a Virtual Water rectangular phantom. The pcsr field proposed by the AAPM Task Group 148 was investigated, performing
ICTR-PHE 2012 mea- surements in the cylindrical Virtual Water phantom supplied by TomoTherapy Inc. Finally, two clinical treatments, head-and-neck and lung, were delivered to the cylindrical phantom to obtain ionization chamber and alanine measurements. The investigation of the CyberKnife system involved the use of PTW31014 (PTW, Freiburg, Germany) and CC13 (IBA Dosimetry GmbH, Germany) ionization chambers. The msr field was defined as the 60 mm diameter field, the largest collimator, at SDD=80 cm. Two pcsr fields were defined, both delivering a homogeneus dose to a 2.5 cm diametre spherical volume in the center of the Solid Water Leksell Gamma Knife Dosimetry Phantom (Elekta AB, Stockholm, Sweden), pcsr1 employing just the 15 mm diameter collimator and pcsr2 using the 15 mm and 20 mm diameter colimators. Two clinical treatments were also investigated, a brain treatment delivered to Leksell phantom, and a lung treatment delivered to 002LFC IMRT Thorax Phantom (CIRS, Norfolk, VA). Results and conclusions: Correction factors obtained in the study of the TomoTherapy unit are shown in table1 with associated uncertainties (k=2). Dose values measured with alanine are systematically lower than those measured with A1SL chamber leading to overall correction factors,kfmsr,fref , kfpcsr,fref and kfclin,fref lower than unity. For the clinical treatments under inves- Qmsr ,Q Qpcsr ,Q Qclin ,Q tigation, clinical-to-msr and clinicalto-pcsr correction factors were found to be compatible with unity, which implies that field factors for relative dosimetry of these clinical fields could be computed simply as msr (pcsr) to clinical chamber readings ratios. Such results indicate that the helical field deliveries in this study do not introduce changes on the ion chamber correction factors for dosimetry. This approach should be considered with caution since only two clinical treatments have been investigated and other treatments could behave differently. Cyberknife measurements were successfully performed and results are currently under investigation.
TABLE I: Correction factors for machine specific reference field, fmsr, plan class specific reference field, fpcsr, and clinical treatments. Associated uncertainties (k=2) affecting last decimals are shown in brackets with two significant digits. 255 HIGH RESOLUTION AND HIGH SENSITIVITY PET IMAGING WITH COMPET M.T. Rissi1, E. Bolle1, D. Volgyes1, O. Dorholt1, K. Hines1, O. Rohne1, S. Stapnes2, A. Skretting3, J. Bjaalie4 1 University of Oslo (NO)
ICTR-PHE 2012 mea- surements in the cylindrical Virtual Water phantom supplied by TomoTherapy Inc. Finally, two clinical treatments, head-and-neck and lung, were delivered to the cylindrical phantom to obtain ionization chamber and alanine measurements. The investigation of the CyberKnife system involved the use of PTW31014 (PTW, Freiburg, Germany) and CC13 (IBA Dosimetry GmbH, Germany) ionization chambers. The msr field was defined as the 60 mm diameter field, the largest collimator, at SDD=80 cm. Two pcsr fields were defined, both delivering a homogeneus dose to a 2.5 cm diametre spherical volume in the center of the Solid Water Leksell Gamma Knife Dosimetry Phantom (Elekta AB, Stockholm, Sweden), pcsr1 employing just the 15 mm diameter collimator and pcsr2 using the 15 mm and 20 mm diameter colimators. Two clinical treatments were also investigated, a brain treatment delivered to Leksell phantom, and a lung treatment delivered to 002LFC IMRT Thorax Phantom (CIRS, Norfolk, VA). Results and conclusions: Correction factors obtained in the study of the TomoTherapy unit are shown in table1 with associated uncertainties (k=2). Dose values measured with alanine are systematically lower than those measured with A1SL chamber leading to overall correction factors,kfmsr,fref , kfpcsr,fref and kfclin,fref lower than unity. For the clinical treatments under inves- Qmsr ,Q Qpcsr ,Q Qclin ,Q tigation, clinical-to-msr and clinicalto-pcsr correction factors were found to be compatible with unity, which implies that field factors for relative dosimetry of these clinical fields could be computed simply as msr (pcsr) to clinical chamber readings ratios. Such results indicate that the helical field deliveries in this study do not introduce changes on the ion chamber correction factors for dosimetry. This approach should be considered with caution since only two clinical treatments have been investigated and other treatments could behave differently. Cyberknife measurements were successfully performed and results are currently under investigation.
TABLE I: Correction factors for machine specific reference field, fmsr, plan class specific reference field, fpcsr, and clinical treatments. Associated uncertainties (k=2) affecting last decimals are shown in brackets with two significant digits. 255 HIGH RESOLUTION AND HIGH SENSITIVITY PET IMAGING WITH COMPET M.T. Rissi1, E. Bolle1, D. Volgyes1, O. Dorholt1, K. Hines1, O. Rohne1, S. Stapnes2, A. Skretting3, J. Bjaalie4 1 University of Oslo (NO)