title: A round-robin gamma stereotactic radiosurgery dosimetry interinstitution comparison of calibration protocols.
abstract: OBJECTIVE
  Absorbed dose calibration for gamma stereotactic radiosurgery is challenging due to the unique geometric conditions, dosimetry characteristics, and nonstandard field size of these devices. Members of the American Association of Physicists in Medicine (AAPM) Task Group 178 on Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance have participated in a round-robin exchange of calibrated measurement instrumentation and phantoms exploring two approved and two proposed calibration protocols or formalisms on ten gamma radiosurgery units. The objectives of this study were to benchmark and compare new formalisms to existing calibration methods, while maintaining traceability to U.S. primary dosimetry calibration laboratory standards.
  
  
  METHODS
  Nine institutions made measurements using ten gamma stereotactic radiosurgery units in three different 160 mm diameter spherical phantoms [acrylonitrile butadiene styrene (ABS) plastic, Solid Water, and liquid water] and in air using a positioning jig. Two calibrated miniature ionization chambers and one calibrated electrometer were circulated for all measurements. Reference dose-rates at the phantom center were determined using the well-established AAPM TG-21 or TG-51 dose calibration protocols and using two proposed dose calibration protocols/formalisms: an in-air protocol and a formalism proposed by the International Atomic Energy Agency (IAEA) working group for small and nonstandard radiation fields. Each institution's results were normalized to the dose-rate determined at that institution using the TG-21 protocol in the ABS phantom.
  
  
  RESULTS
  Percentages of dose-rates within 1.5% of the reference dose-rate (TG-21+ABS phantom) for the eight chamber-protocol-phantom combinations were the following: 88% for TG-21, 70% for TG-51, 93% for the new IAEA nonstandard-field formalism, and 65% for the new in-air protocol. Averages and standard deviations for dose-rates over all measurements relative to the TG-21+ABS dose-rate were 0.999±0.009 (TG-21), 0.991±0.013 (TG-51), 1.000±0.009 (IAEA), and 1.009±0.012 (in-air). There were no statistically significant differences (i.e., p>0.05) between the two ionization chambers for the TG-21 protocol applied to all dosimetry phantoms. The mean results using the TG-51 protocol were notably lower than those for the other dosimetry protocols, with a standard deviation 2-3 times larger. The in-air protocol was not statistically different from TG-21 for the A16 chamber in the liquid water or ABS phantoms (p=0.300 and p=0.135) but was statistically different from TG-21 for the PTW chamber in all phantoms (p=0.006 for Solid Water, 0.014 for liquid water, and 0.020 for ABS). Results of IAEA formalism were statistically different from TG-21 results only for the combination of the A16 chamber with the liquid water phantom (p=0.017). In the latter case, dose-rates measured with the two protocols differed by only 0.4%. For other phantom-ionization-chamber combinations, the new IAEA formalism was not statistically different from TG-21.
  
  
  CONCLUSIONS
  Although further investigation is needed to validate the new protocols for other ionization chambers, these results can serve as a reference to quantitatively compare different calibration protocols and ionization chambers if a particular method is chosen by a professional society to serve as a standardized calibration protocol.
authors.affiliation: Department of Radiation Oncology, Washington University, St. Louis, Missouri 63110.
authors.forename: R E
authors.initials: RE
authors.lastname: Drzymala
authors.affiliation: Imaging and Radiation Oncology Core Houston, UT MD Anderson Cancer Center, Houston, Texas 77030.
authors.forename: P E
authors.initials: PE
authors.lastname: Alvarez
authors.affiliation: Radiation Oncology Department, University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania 15232.
authors.forename: G
authors.initials: G
authors.lastname: Bednarz
authors.affiliation: Department of Radiation Oncology, Wake Forest University, Winston-Salem, North Carolina 27157.
authors.forename: J D
authors.initials: JD
authors.lastname: Bourland
authors.affiliation: Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: L A
authors.initials: LA
authors.lastname: DeWerd
authors.affiliation: Department of Radiation Oncology, University California San Francisco, San Francisco, California 94143.
authors.forename: L
authors.initials: L
authors.lastname: Ma
authors.affiliation: Department of Radiation Oncology, Duke University Medical Center, Durham, North Carolina 27710.
authors.forename: S G
authors.initials: SG
authors.lastname: Meltsner
authors.affiliation: Department of Radiation Oncology, The Cleveland Clinic Foundation, Cleveland, Ohio 44195.
authors.forename: G
authors.initials: G
authors.lastname: Neyman
authors.affiliation: Medical Physics Department, Hospital Na Homolce, Prague 15030, Czech Republic.
authors.forename: J
authors.initials: J
authors.lastname: Novotny
authors.affiliation: Gamma Knife Center, Washington Hospital Healthcare System, Fremont, California 94538.
authors.forename: P L
authors.initials: PL
authors.lastname: Petti
authors.affiliation: Department of Radiation Oncology, Tufts University School of Medicine, Boston, Massachusetts 02111.
authors.forename: M J
authors.initials: MJ
authors.lastname: Rivard
authors.affiliation: Department of Radiation Oncology, University of Southern California, Los Angeles, California 90033.
authors.forename: A S
authors.initials: AS
authors.lastname: Shiu
authors.affiliation: San Diego Medical Physics, Inc., La Jolla, California 92037.
authors.forename: S J
authors.initials: SJ
authors.lastname: Goetsch
mesh_terms: D000388:Air; D002138:Calibration / Q000592:standards; D002985:Clinical Protocols / Q000592:standards; D005720:Gamma Rays / Q000627:therapeutic use*; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods* / Q000592:standards; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D011879:Radiotherapy Dosage; D014481:United States; D014867:Water
chemical_list: D014867:Water