SU‐F‐T‐124: Radiation Biological Equivalent Presentations OfLEM‐1 and MKM Approaches in the Carbon‐Ion Radiotherapy

Purpose: To study the correlations of the radiation biological equivalent doses (BED) along depth and lateral distance between LEM‐1 and MKM approaches. Methods: In NIRS‐MKM (Microdosimetric Kinetic Model) approach, the prescribed BED, referred as C‐Eq, doses aims to present the relative biological effectiveness (RBE) for different energies of carbon‐ions on a fixed 10% survival value of HCG cell with respect to convention X‐ray. Instead of a fixed 10% survival, the BED doses of LEM‐1 (Local Effect Model) approach, referred as X‐Eq, aims to present the RBE over the whole survival curve of chordoma‐like cell with alpha/beta ratio of 2.0. The relationship of physical doses as a function of C‐Eq and X‐Eq doses were investigated along depth and lateral distance for various sizes of cubic targets in water irradiated by carbon‐ions. Results: At the center of each cubic target, the trends between physical and C‐Eq or X‐Eq doses can be described by a linear and 2nd order polynomial functions, respectively. Using fit functions can then calculate a scaling factor between C‐Eq and X‐Eq doses to have similar physical doses. With equalized C‐Eq and X‐Eq doses at the depth of target center, over‐ and under‐estimated X‐Eq to C‐Eq are seen for depths before and after the target center, respectively. Near the distal edge along depth, sharp rising of RBE value is observed for X‐Eq, but sharp dropping of RBE value is observed for C‐Eq. For lateral locations near and just outside 50% dose level, sharp raising of RBE value is also seen for X‐Eq, while only minor increasing with fast dropping for C‐Eq. Conclusion: An analytical function to model the differences between the CEq and X‐Eq doses along depth and lateral distance need to further investigated to explain varied clinic outcome of specific cancers using two different approaches to calculated BED doses.