Room: Exhibit Hall | Forum 4
Purpose: 4D-optimization integrates motion into the optimization process for â€˜motion-robustâ€™ PBS plans. We explore the limits of this approach through a systematic evaluation to benchmark its effectiveness as a motion mitigation approach.
Methods: 192 4DCTs were generated of spherical targets (HU=100) moving in water according to sin4 pattern, using 4 target-sizes (diameter=20/40/60/80mm) and motions in 4 peak-to-peak-amplitudes (5/10/20/30mm) and 6 periods (3/4/5/6/7/8s), assuming anatomy with/without bony-structure (diameter=16mm) in the field entrance. Single-field 3D-plans were optimized for both CTV (end-of-exhalation-phase) and geometrical-ITV (encapsulated-CTVs-of-all-phases). 4D dose calculations were performed under motion conditions by assuming single and x10 layered-rescanning. 4D-optimized-plans were derived from 4D-recalculated 3D-optimized-plans using 4D-optimization which directly considered delivery dynamics, target motion and induced density variation, including rescanning into the optimisation process. All plans were quantified using homogeneity index of D5-D95 inside CTV, and mean doses in surrounding non-target tissues.
Results: Comparable D5-D95 in CTV (â‰¤10% of static) was achieved for 76/192 cases (10mm-motion) by 4D optimization alone. When combining 4D optimization with rescanning, 121/192 cases can also reach the goal. However, 4D optimization is less effective for the 48 scenarios with the highest amplitudes (30mm), or for the 18 scenarios on the smallest target (d=20mm) when density heterogeneity was included. For the successful cases (63%), significant reduction of the mean dose to surrounding (8-20%) is obtained using 4D-optimisation on CTV comparing to 3D-optimised plans on gITV. Resultant beam weights after 4D-optimization were in the range of 20-400% (5th-95th percentile) of the initial values, which are deliverable if the proton beam current is adjusted accordingly.
Conclusion: The effectiveness of 4D optimization is strongly correlated to motion amplitude and density heterogeneity, but less to motion period. 4D optimization can achieve sufficient target dose distribution, while minimizing gITV margins for normal issue dose reduction. Its advantages are further enhanced once combined with rescanning.
Funding Support, Disclosures, and Conflict of Interest: The work was partially supported by the research funding from Varian medical Systems - Particle Therapy GmbH, Trosidorf, Germany