Purpose: To analyze the accuracy of relative biologic effectiveness (RBE) values for treatment planning in carbon ion radiotherapy based on the local effect model (LEM) and to discuss the implications on the clinically relevant depth dose profiles.
Methods and materials: Predictions of the LEM are compared with a broad panel of experimental data in vitro and to the tolerance of the rat spinal cord in vivo. To improve the accuracy of the LEM, the description of track structure is modified by taking into account a velocity-dependent extension of the inner part of the track.
Results: The original version of the LEM (LEM I) underestimates the therapeutic ratio of carbon ions (i.e., the ratio of RBE in the Bragg peak region as compared with the RBE in the entrance channel). Although significantly reduced, the cluster extension of the LEM (LEM II) still shows the same tendency. Implementation of the modified track structure (LEM III) almost completely compensates these systematic deviations, and predictions of RBE by LEM III for high and low energetic carbon ions show good agreement for a wide panel of different cell lines, as well as for the tolerance of the rat spinal cord. As a consequence, the expected RBE in the normal tissue surrounding the tumor becomes significantly lower than estimated with the LEM in its original version (LEM I).
Conclusions: The modified track structure description represents an empiric approach to improve the accuracy of the LEM for treatment planning. This will be particularly useful for further optimization of carbon ion therapy in general and with respect to comparison with other treatment modalities, such as protons or intensity-modulated radiotherapy.