RBE of kV CBCT radiation determined by Monte Carlo DNA damage simulations

Area of Science:

• Medical Physics
• Radiation Biology
• Computational Biology

Background:

• Kilovoltage (kV) radiation, used in cone beam computed tomography (CBCT), has a higher linear energy transfer (LET) than megavoltage sources.
• This higher LET suggests a potential for increased relative biological effectiveness (RBE) and DNA double-strand breaks (DSBs).

Purpose of the Study:

• To develop and validate a computational method for accurately predicting DSB yields and RBE from kV CBCT radiation.
• To quantify the RBE of kV CBCT X-rays for DSB induction and assess the influence of depth, filtration, and oxygen concentration.

Main Methods:

• Coupling the Monte Carlo (MC) radiation transport code PENELOPE with the Microdosimetry and Damage Simulation (MCDS) code.
• Implementing a novel tally algorithm in PENELOPE to track incident electrons on a cell nucleus and record their energy and deposited dose.
• Using MCDS to simulate DSB yields for monoenergetic electrons and weighting these by the energy spectrum obtained from PENELOPE.

Main Results:

• The study predicts an RBE of 1.1 for DSB induction by diagnostic X-rays (80-125 kVp).
• No significant variation in RBE was observed with changes in depth or filtration.
• Absolute DSB yields were approximately three times lower under anoxic conditions compared to normoxic or aerobic conditions, but oxygen concentration had a negligible effect on RBE (± 0.02).

Conclusions:

• The developed MC-MCDS coupling method accurately predicts DSB induction and RBE for kV CBCT radiation.
• kV CBCT X-rays exhibit a modest RBE of 1.1 for DSB induction, with oxygen concentration not significantly affecting this RBE value.
• The findings contribute to a better understanding of the biological impact of kV CBCT, aiding in dose optimization and risk assessment.