AAPM TG178: Towards the Recommendation of a Standard Dosimetry Protocol for Gamma Knife Stereotactic Radiosurgery Paula L. Petti, PhD Washington Hospital Healthcare System Fremont, California, USA
AAPM Task Group 178 Members Paola Alvarez, U. TX, MD Anderson Greg Bednarz, U. Pittsburgh J. Daniel Bourland, Wake Forest U. Larry DeWerd, U. Wisconsin Madison ADCL Robert Drzymala, Washington University, St. Louis, MO Steven Goetsch, San Diego GK Center (Task group chair) David Larson, Washington Hospital, Fremont, CA Lijun Ma, University California, San Francisco Sheridan Meltsner, Duke Univ. Med. Ctr Gennady Neyman, Cleveland Clinic Josef Novotny, Hospital Na Homolce, Prague, Czech Republic Paula Petti, Washington Hospital, Fremont, CA Mark Rivard, Tufts Univ. School of Med. Arjun Sahgal, Sunnybrook Research Institute, Toronto, Ontario, Canada Almon Shiu, Univ. Southern California
AAPM Task Group 178 Consultants Jonas Johansson, Elekta Instrument AB, Stockholm, Sweden Keith Kunugi, Univ. Wisconsin, Madison, WI Jan Seuntjens, McGill Univ, Montreal, Quebec, Canada
Disclosures Paula Petti, Gennady Neyman, Steven Goetsch and Josef Novotny are consultants for Elekta Instrument AB
Importance of a Standardized Dosimetry Protocol Promote consistent dose calibration procedures across all GK institutions Reduce probability of dose delivery error Challenge: Gamma Knife a unique system Calibration protocols specific to linear accelerators not necessarily valid
Background: Dose Calibration Protocols Ionization chamber calibrated in air Parameters AAPM TG-21 (1983) Meltsner-DeWerd (2009)* Tabulated based on measurements Monte-Carlo (MC) generated Advantages Not specific to one type of phantom or to one phantom material. Phantom can be water, TE plastic, etc. Not specific to linear accelerator Disadvantages Multiple steps in the calculation Complexity could lead to errors MC parameters need to be calculated for each ion chamber Requires that the user purchase special equipment *Med. Phys. 36, 339-350 (2009)
Background: Dose Calibration Protocols Dosimetry protocols for linear accelerators have evolved away from methods that use an in-air ionization chamber calibration factor and towards those such that the ionization chamber is calibrated in water For in-water-calibrated ionization chambers, only a few simple multiplicative factors are required to convert the ionization chamber reading to dose-to-water.
Background: Dose Calibration Protocols Ionization chamber calibrated in water IAEA TRS-398 (2000) AAPM TG-51 (1999) IAEA Working Group (Alfonso 2008)* Parameters Simple measured Simple measured Simple measured + Monte Carlo (MC) generated Advantages Simple to use Machine-specific correction factor Accounts for different ionization chambers and phantom types Disadvantages *Med. Phys. 35, 5179-5186 (2008) Designed for use with linear accelerators: 10 x 10 cm2 field Water phantom MC parameters must be generated for each ionization chamber and phantom combination
Round-robin dosimetry inter-comparison 9 participating institutions; 10 GK units (3 Model 4C and 7 PFX) Each institution performed at least 8 sets of measurements: 3 different spherical phantoms 1 in-air jig 2 different ionization chambers Compared 4 dosimetry protocols/formalisms TG-21 Meltsner-DeWerd In-air TG-51 IAEA Alfonso formalism Results published: Drzymala, et al., Med. Phys. 42(11), 6745-56, 2015
Equipment and Measurements Elekta ABS Elekta solid water (SW) Phantoms Ionization Chambers PTW TN31010 (PTW) Standard Imaging Exradin A16 (Ex) Electrometer: Collecting Volume 0.125 cc 0.007 cc Standard Imaging Max 4000 Phantom laboratories liquid water (LW) Standard Imaging in-air jig In-air measurement
Normalization of Data To compare data from each institution, dose rates calculated with each protocol were normalized to the dose rate determined by that institution in the ABS phantom using the TG-21 protocol. This normalization was chosen because historically the ABS has been the most commonly used phantom in GK dosimetry, and TG-21 is currently the only protocol directly applicable to GK for the 3 phantom types considered.
Results Averaged over all Institutions Sorted by Dosimetry Protocol/Formalism
Results Averaged over all Institutions Sorted by Dosimetry Protocol/Formalism
Results Averaged over all Institutions Sorted by Dosimetry Protocol/Formalism
Comparing relative advantages of viable protocols TG-21 MDW In-air New IAEA Alfonso Ionization chamber calibration is in terms of absorbed dose to water Can be used for different phantom materials NA User need not purchase new equipment Requires only a few simple calculations Protocol parameters readily available in literature
Comparing relative advantages of viable protocols TG-21 MDW In-air New IAEA Alfonso Ionization chamber calibration is in terms of absorbed dose to water Can be used for different phantom materials NA User need not purchase new equipment Requires only a few simple calculations Protocol parameters readily available in literature
ff mmmmmm,ff rrrrrr IAEA (Alfonso) Formalism kk QQmmmmmm,QQ factors from Elekta Phys. Rep.* Take a closer look *Johansson and Gorka, Elekta Physics Report: Reference No. SSM 2010/2201, Project nr 4017003-006)
ff Alfonso kk mmmmmm,ff rrrrrr factors for PFX from Elekta Phys. Rep. QQmmmmmm,QQ
ff Alfonso kk mmmmmm,ff rrrrrr factors for PFX from Elekta Phys. Rep. QQmmmmmm,QQ For IC Vol > 0.05 cm3 1.000 ± 0.0015 1.004 ± 0.0014 1.012 ± 0.0034
Potential Recommendation for Perfexion 1) Ionization chamber: If possible, select a thimble ionization chamber with collecting volume > 0.05 cc, but small enough to satisfy smallfield conditions (active cylindrical volume < 7 mm length, 5 mm diameter) 2) Phantom: Select 8-cm radius spherical phantom based on personal preference 3) Protocol: New IAEA Alfonso Formalism* with parameters shown on next slide - *TG-21 remains a reasonable option, but not optimal
Potential Recommendation for new IAEA Alfonso formalism Perfexion 1) For ionization chambers with collecting volume 0.05 cm 3, * ff mmmmmm,ff rrrrrr kk QQmmmmmm,QQ = 1.000 for liquid water phantom 1.004 for Elekta Solid Water phantom 1.012 for Elekta ABS phantom 2) For ionization chambers with collecting volume < 0.05 cm 3, ff mmmmmm,ff rrrrrr kk QQmmmmmm,QQ = Tabulated value from Elekta Phys. Report, if available Value determined from new M.C. calculations *Contingent upon corroboration of K-factors by other investigations, and upon the publication of these data
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