PO03: Evaluation of DNA Damage Induced by the Xoft Axxent Source

Abstract

<h3>Purpose</h3> To evaluate the level of DNA double strand breaks (DSB) induced by the Xoft Axxent source compared to megavoltage beam qualities for cervical cancer applications. <h3>Materials and Methods</h3> HeLa cells were suspended in DMEM media in 1.5 mL microcentrifuge tubes and irradiated at doses of 0, 2, 4, and 6 Gy using the bare Axxent source (BS) and source in titanium applicator (SIA). Additionally, HeLa cells suspensions in 15 mL centrifuge tubes were irradiated in ⁶⁰Co. Following irradiation, cells were embedded in LMAgarose gel on two well slides and neutral comet assays were run. Cells were stained with SYBR Gold stain, imaged using fluorescence microscopy, and the olive tail moment (OTM) was calculated using CometScore 2.0 software to quantify the number of DSBs. The TOPAS nBio extension was also used to evaluate the damage to a cell nucleus for the three beam qualities. The energy spectrum of the secondary electrons produced in the tubes of cells was scored and the average energies were calculated. Monoenergetic electron sources with these energies were placed in close proximity to a cell with a nucleus with a radius of 4 μm and the multiple nucleus scorers available with the extension were used. The dose rate of the sources was not considered in these simulations. <h3>Results</h3> A significant difference was observed between the BS and SIA at the 6 Gy dose point. A nonsignificant increase in OTM at 2 Gy and 4 Gy was observed between the BS and SIA. Further, a statistically significant difference was observed between the BS and 60Co at all dose points and between the SIA and 60Co at 2 Gy and 4 Gy. A nonsignificant difference was observed between the latter two beam qualities at 6 Gy, where there was a small increase in OTM for the SIA. When comparing to the nBio simulations, there was no significant difference observed between the DNA damage caused by the BS and SIA. The discrepancies between the experiments and simulations indicate the dose rate of the radiation plays a significant role in the damage induced to the DNA strand. <h3>Conclusions</h3> The reduced dose rate of the SIA plays a significant role in the differences observed between the BS and SIA DNA damage. Overall, both Axxent beam qualities provided an advantage of DSB induction compared to megavoltage beam qualities and indicate an advantage of using these sources clinically. Care should be taken to understand the limitations the reduced dose rate has on the resulting biological response to radiation.