Impact of decaying dose rate in gamma knife radiosurgery: in vitro study on 9L rat gliosarcoma cells

Purpose: Leksell Gamma Knife (LGK) installations replace their Co-60 sources every 5-10 years corresponding to one - two Co-60 half-lives. Between source replacements the dose rate gradually declines. The purpose of this study was to assess whether the decreasing dose rates associated with radioactive decay of Co-60 may affect the radiobiological response of a given dose delivered to 9L rat gliosarcoma cells. Method and Materials: 9L rat gliosarcoma cells were irradiated using LGK U, LGK 4C, and LGK Perfexion providing 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 using each of the LGK units, the irradiated cells were plated into petri dishes. Two weeks later cell colonies with greater than 50 cells were counted. The survival of cells was plotted as a function of dose over the range of delivered doses and fitted to a linear quadratic function of the form S-D = e(-alpha D)-(beta D2), where a and beta are terms fit using the Levenberg-Marquardt algorithm. Conclusions: This study demonstrated no difference in tumor cell kill in the range of dose rates when using actual LGK unit with new sources or with sources decayed even for two half lives. This study focused on tumor cells. In future studies we will reassess the dose rate effect on cultured neurons to simulate response of normal healthy brain tissue to different dose rates.