To identify suitable combination strategies for treatment planning and beam delivery in scanned carbon ion therapy of moving tumors.
Approach: Carbon ion treatment plans for five abdominal tumors were optimized on 4DCT data using the following approaches: 4DITV across all phases and within a gating window, single phase uniform dose (SPUD), and an innovative 4D tracking ITV strategy. Delivered single-fraction doses were calculated on time-resolved virtual CT images reconstructed from 2D cine-MRI series, using a Deformable Image Registration pipeline. Treatment plans were combined with various beam delivery techniques: 3D (no motion mitigation), rescanning, gating, beam tracking, and multi-phase 4D delivery with and without residual tracking (MP4D and MP4DRT) to form in total 11 treatment modalities. Single fraction doses were accumulated to simulate a fractionated treatment.
Main results: Breath-sampled treatments using the MP4D and MP4DRT delivery techniques were the only to achieve D95 >
95 % for hypofractionated treatments, with little dependence on the number of fractions. A combination of MP4DRT with the new 4D tracking ITV approach resulting in conformal dose distributions and demonstrated the greatest robustness against irregular motion and anatomical changes.
Significance: This study demonstrates, that real-time adaptive beam delivery strategies can deliver conformal doses within single fractions, thereby enabling hypofractionated treatment schemes that are not feasible with conventional strategies.