Impact Of Dose Rate In Gamma Knife Radiosurgery

Keywords: Dose rate, gamma knife, Dose, outcome, dose planning

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     Leksell Gamma Knife (LGK) installations replace their Co-60 sources every 5-10 years corresponding to one - two Co-60 half-lives of the isotope that emits Gamma rays. Between source replacements the dose rate gradually declines.   
     The purpose of this study was to assess whether different dose rates may affect the radiobiological response of a given dose delivered using a cell culture technique.
     9L rat gliosarcoma cells were placed a at the center of an ELEKTA ABS spherical phantom. Three LGK units( LGK U, LGK 4C, and LGK Perfexion ) provided three different dose rates of 0.770 Gy/min (sources reloaded 12.0 years ago), 1.853 Gy/min (sources reloaded 5.0 years ago) and 2.937 Gy/min (sources reloaded 1.6 years ago), respectively. After irradiation of cell samples to 4.0 Gy, 8.0 Gy and 16.0 Gy on each of the LGK units, the irradiated cells were plated on well plates that have been pre-plated with feeder cells to enhance plating efficiency. Two weeks after plating, the colonies were fixed with 70% alcohol and stained with Meyer’s hematoxylin. Those cell colonies with greater than 50 cells were counted, and from the cell colonies of irradiated and un-irradiated reference cells, the survival of irradiated cells was calculated. Survival was plotted as a function of dose over the range of delivered doses and fitted to a linear quadratic function of the form SD = e-aD-ßD², where a and ß are terms fit using the Levenberg-Marquardt algorithm.
     Significantly smaller number of formed cell colonies can be observed for cells irradiated by 16.0 Gy dose. Cells from the 16.0 Gy sample were plated at a concentration 120X higher than non-irradiated cells. No significant difference is observed for survival in the range of dose rates used in this study. Three LGK units with different dose rates used in this study represented a useful range of dose rates used in a clinical practice.
     This is a retrospective study.
     This finding supports the hypothesis that there is no difference in radiobiological response of 9L gliosarcoma cells using LGK units with dose rates that vary between 0.770 Gy/min and 2.937 Gy/min.
     In future studies we will reassess the dose rate effect on cultured neurons.


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