Motion mitigation in scanned ion beam therapy through 4D-optimization.

The treatment of moving tumors remains challenging, especially with scanned ion beam therapy due to interplay effects and the strong range dependence. This is especially true in the context of radiosurgery with high dose delivered in few or single fractions. Inverse treatment planning on the entire 4D-CT may result in conformal plans inherently adapted to the moving anatomy of the patient. Existing studies on this topic for photon therapy are reviewed, but arguably the benefits for ion beam therapy can be even greater. Compared to the main conformal mitigation technique of beam tracking, 4D-optimization permits a) easier, offline handling of range changes, b) handling of complex motion patterns, and c) improved dose shaping capabilities outside of the target. Different approaches for 4D-optimization in scanned ion beam therapy are proposed and compared, together with delivery methods that provide the necessary synchronization between irradiation and detected patient motion. Potential solutions for the improvement of robustness in 4D-optimization are discussed. A method for delivery of homogenous doses to each motion phase is presented that might be a potential solution for robust conformal dose delivery for future clinical use. In an exemplary lung cancer patient case with a large motion amplitude, 4D-optimization resulted in conformal dose coverage while beam tracking did not. In conclusion, different strategies of 4D-optimization could provide increased OAR sparing and highly conformal dose delivery for targets with complex motion patterns and large amplitudes.