Evaluation of Motion Mitigation Using Abdominal Compression in the Clinical Implementation of Pencil Beam Scanning Proton Therapy of Liver Tumors

Purpose: To determine whether individual liver tumor patients can be safely treated with pencil beam scanning proton therapy. This study reports a planning preparation workflow that can be used for beam angle selection and the evaluation of the efficacy of abdominal compression (AC) to mitigate motion. Methods: Four-dimensional computed tomography scans (4DCT) with and without AC were available from 10 liver tumor patients with fluoroscopy-proven motion reduction by AC. For each scan, the motion amplitudes and the motion-induced variation of water equivalent thickness (ΔWET) in each voxel of the target volume were evaluated during treatment plan preparation. Optimal proton beam angles were selected after volume analysis of the respective beam-specific planning target volume (BSPTV). M₈₀ and ΔWET₈₀ derived from the 80ᵗʰ percentiles of motion amplitude (M) and ΔWET were compared with and without AC. Proton plans were created on the average CT. 4D dynamic dose calculation was performed post plan by synchronizing proton beam delivery timing patterns to the 4DCT phases to assess interplay and fractionation effects, and to determine motion criteria for subsequent patient treatment. Results: AC resulted in reductions in mean Liver-GTV dose, M, ΔWET, and BSPTV volumes and improved dose degradation (ΔD₉₅ and ΔD₁) within the CTV. For small motion (M₈₀ < 7 mm and ΔWET₈₀ < 5 mm), motion mitigation was not needed. For moderate motion (M₈₀ 7-10 mm or ΔWET₈₀ 5-7 mm), AC produced a modest improvement. For large motion (M₈₀ > 10 mm or ΔWET₈₀ > 7 mm), AC and/or some other form of mitigation strategies were required. Conclusion: A workflow for screening patients’ motion characteristics and optimizing beam angle selection was established for the pencil beam scanning proton therapy treatment of liver tumors. Abdominal compression was found to be useful at mitigation of moderate and large motion.