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Energy Layer Filtering and Splitting in Breath Sampled Re-Painting for Lung SBRT Using Pencil Beam Scanning Proton Beams

K Stiles1*, C Chang1, A Stanforth1,2, Y Wang1, J Demoor2, K Higgins1, M McDonald1, J Bradley1, T Liu1, X Yang1, L Lin1, J Zhou1, (1) Emory University, Atlanta, GA, (2) Georgia Institute Of Technology, Atlanta, GA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The breath-sampled repainting is the most effective repainting scheme for lung SBRT using a pencil beam scanning (PBS) proton system. We have developed an energy-layer filtering and splitting technique to efficiently optimize energy-layers and repaint each layer within a breathing cycle to reduce the motion interplay effect in lung SBRT.

Methods: An energy-layer filtering algorithm was developed to select the most effective layers with substantial contributions to the plans. Layer repainting was implemented by splitting the optimized layers into 4 adjacent layers (±0.2MeV). A 4D-CT scan (10 phases) of a CIRS lung motion phantom with 3cm insert was conducted with sinusoidal motion of 1.5cm and 2.0cm in the superior-inferior direction. Three plans were created: non-4D optimized plan, 4D optimized plan, and the proposed energy-layer selection repainting (ESRP) plan, all normalized to CTV V100=49Gy. An interplay evaluation tool was used to calculate the dosimetric effect from motion period (4s vs 8s) and starting phases (phase 0 vs 50). Dosimetric parameters (CTV D99, V110, and Dmax) were calculated. Gafchromic film was sandwiched in the phantom insert, and the plans were delivered on a PBS proton machine with corresponding parameters. Gamma analysis was performed using a 7%/7mm criteria.

Results: The interplay evaluation of CTV D99 for the corresponding three plans were calculated to be 42.7±1.9Gy vs. 47.3±0.5Gy vs. 48.0±0.2Gy (p<0.05), respectively. The corresponding V110 and Dmax were calculated to be 33.4±7.1% vs. 49.7±2.4% vs. 79.1±8.0% (p<0.05) and 60.5±0.4Gy vs. 62.6±0.9Gy vs. 59.3±0.5Gy (p<0.05), respectively. Gamma analysis passing rates from the measurements were found to be 59.7±3.9%, 70.4±5.5%, and 84.5±3.9% respectively.

Conclusion: The evaluation tool validated that the ESRP method mitigated motion of the tumor better than 4D optimization, and was verified from film measurements. The evaluation tool overestimated the effect 4D optimization had on mitigating motion due to assumed set beam parameters.


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