Relative output factor and beam profile measurements for the Leksell Gamma Knife Perfexion over a 4 year time period

Paula Petti1, Sandeep Kunwar2, David Larson3

1Fremont, United States 2Taylor McAdam Bell Neuroscience Institut, Washington Hospital Healthcare System, 3San Francisco, CA United States

Keywords: physics, Dose, gamma knife, Dose Prescription, radiosurgery

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     Recently, a new version of Leksell GammaPlan (LGP10) was introduced with updated relative output factors (ROF) and beam profiles. The most notable change in parameter values between LGP9 and LGP10 was for the 8-mm ROF for the Leksell Gamma Knife® (LGK) Perfexion™, which decreased from 0.924 to 0.900. Other parameters, e.g., the 4-mm Perfexion™ ROF, the full-width-half-maximum (FWHM) and 80%-20% penumbrae, changed to a lesser degree (by 1% for the 4-mm ROF, by £ 0.4 mm for any of the FWHM and by £ 0.25 mm for the penumbrae).
     The purpose of this paper is to compare measurements performed as part of our annual quality assurance (QA) program to modeled data in LGP.
     Since the installation of the LGK Perfexion™ at our institution in 2007, we have conducted 4 annual QA assessments (2008 – 2011). Relative output factors for the 4- and 8-mm collimators and beam profiles for all collimators were measured by irradiating EBT or EBT2 GAFCHROMIC film in the Leksell spherical ABS plastic phantom. A calibration curve was obtained for the film, and film-handling procedures recommended by the manufacturer were followed. The film was scanned at 400 dpi using an Epson Perfection V700 scanner and analyzed with the U.S. National Institute of Health ImageJ software.
     Average ROF values over the 4-year time period for the 8- and 4-mm collimators were 0.889 ± 0.012 and 0.782 ± 0.014. These measurements agree with the values in LGP10 to within 1% for the 8-mm collimator and to within 4% for the 4-mm collimator. The agreement between measured and modeled values for the FWHM for all 3 collimators ranged from -0.2 to 0.4 mm for LGP9 and from -0.5 to 0.2 mm for LGP10. Measured penumbra values agreed with those modeled in LGP9 or LGP10 to within 0.5 mm and 0.3 mm respectively.
     Annual QA measurements at our institution support the LGP10 updated ROF for the Perfexion™ 8-mm collimator.
     Because changes in the 4-mm Perfexion ROF and beam profiles were relatively small, our measured data agree with either the newer or older LGP values to within experimental uncertainty.


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P. Petti (), S. Kunwar (), D. Larson ()