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Dosimetry and mechanical accuracy of the first rotating gamma system installed in North America
Author(s) -
Kubo Hideo D.,
Araki Fujio
Publication year - 2002
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1514039
Subject(s) - collimator , dosimetry , imaging phantom , nuclear medicine , calibration , thermoluminescent dosimeter , ionization chamber , radiosurgery , linear particle accelerator , truebeam , optics , physics , materials science , dosimeter , beam (structure) , medical physics , medicine , radiation therapy , ion , quantum mechanics , ionization
The purpose of this paper is to present the dosimetry and mechanical accuracy of the first rotating gamma system (RGS) installed in North America for stereotactic radiosurgery. The data were obtained during the installation, acceptance test procedure, and commissioning of the unit. The RGS unit installed at UC Davis Cancer Center ( RGS u ) has modifications on the source and collimator bodies from the earlier version of the Chinese RGS ( RGS c) . The differences between these two RGSs are presented. The absolute dose at the focal point was measured in a 16‐cm‐diam acrylic phantom using a small volume chamber, which was calibrated at the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UW‐ADCL). The dose in acrylic was then converted to a dose in water. A collimator output factor from each of the four different collimator sizes ranging from 4, 8, 14, and 18 mm was measured with (1) a smaller volume chamber and (2) approximately 3 . 0 mm × 3.0 mm × 1.0 mm TLD chips in the same acrylic phantom. The Gafchromic films were used for the dose profile, collimator output factor, and mechanical/radiation field isocentricity measurements. The TLD chips were processed in‐house whereas Gafchromic films were processed both at the UW‐ADCL and in‐house. The timer error, timer accuracy, and timer linearity were also determined. The dose profiles were found to be similar between RGS cand RGS u . The 4 mm collimator output factor of the RGS uwas approximately 0.6, similar to that from RGS c , in comparison to 0.8 in the report for a Leksell Model U Gamma‐Knife. The mechanical/radiation field isocentricity for RGS cand RGS uis found to be similar and is within 0.3 mm in both X and Y directions. In the Z direction, the beam center of the RGS uis shifted toward the sources by 0.75 mm from the mechanical isocenter whereas no data are available for RGS c . Little dosimetric difference is found between RGS uand RGS c . It is reported that RGS chas the same dosimetric and mechanical characteristics as the Model U Gamma‐Knife. Therefore, RGS ushould be capable of achieving stereotactic radiosurgery with the same degree of dosimetric and mechanical accuracy as with the Gamma‐Knife.