Purpose: To measure the relative biological effectiveness (RBE) of the Xoft Axxent electronic brachytherapy source on HeLa cells with the bare S7600 Axxent source (BS) and source-in-titanium applicator (SIA).
Methods: HeLa cells were suspended in 1 mL of cell culture media and placed into 1.5 mL microcentrifuge tubes. Cells were irradiated at a radial distance of 2 cm from the source and given doses ranging from 0-6 Gy for both the BS and SIA. The source was pulled back a distance of 3 mm for the SIA irradiations. Doses from the BS and SIA were determined using an Exradin A26 ionization chamber, TLD-100 microcubes, and the TOPAS Monte Carlo code. Irradiations were also performed in Co-60 using 15 mL tubes placed in a virtual water phantom at an SAD of 75 cm and depth of 1.5 cm to obtain reference dose points. Following irradiation, each suspension was seeded in a 6-well plate, with an equal concentration of cells per well. The plates were incubated for 1 week, stained with crystal violet, and the surviving colonies were counted. Survival curves were plotted and fit using a linear-quadratic equation.
Results: The RBE values determined for the BS were found to be 1.19, 1.22, and 1.27 for the surviving fractions of 1%, 10%, and 37% respectively. The RBE determined for the SIA was found to be 1.17, 1.19, and 1.22 for the surviving fractions of 1%, 10%, and 37% respectively.
Conclusion: The RBE of the Axxent source is approximately 20% greater than that of Co-60, which is representative of current external beam treatments. Additionally, filtration of the Axxent source from the applicator causes a reduction in dose rate and small reduction in the RBE of the Axxent source.
Funding Support, Disclosures, and Conflict of Interest: The sources and applicator used for this work were provided by Xoft, a subsidiary of iCAD Inc.