Relative biological effectiveness study of Lipiodol based on microdosimetric-kinetic model

•We revealed the RBE enhancement with the physical dose, energy fluence and biological parameter.•RBE with FFF beam was larger than that with FF beam.•D10% and D1% in the Lipiodol were larger than those in water.•RBE enhancement was larger than physical dose enhancement in the Lipiodol. We examine the contrast agent Lipiodol effect on the relative biological effectiveness (RBE) values for flattening filter free (FFF) and flattening filter (FF) beams of 6 MV-Xray (6 MVX) and 10 MVX. Lipiodol was placed at 5 cm depth in water. According to the microdosimetric kinetic model, the RBE values for killing the human liver hepatocellular cells were calculated from dose and lineal energy (yd(y)) from Monte Carlo simulations. RBE200kVX and RBECo were defined as the ratios of dose using reference radiation (200 kVX, Co-ɤ) to the dose of test radiation (FFF and FF beams for 6 MV and 10 MV) to produce the same biological effects. The dose enhancement RBE (RBEDE) was defined as the ratios of a dose without Lipiodol to with Lipiodol using to produce the same biological effects. The dose needed to achieve 10% (D10%) and 1% cell survival (D1%) was evaluated by cell surviving fraction (SF) formula. The deviation of mean y‾D values with and without Lipiodol were 3.9–4.8% for 6 MVX and 3.5–3.6% for 10 MVX. The RBE200kVX and RBECo with Lipiodol were larger than that without Lipiodol. The RBEDE was larger for FFF beam than for FF beam. The deviation of RBEDE for FFF and FF beams of 6 MVX was larger than that of 10 MVX. The presence of Lipiodol seemed to locally increase the absorbed dose and to also cause an enhancement of the relative biological effectiveness.