ESTRO 2021 Abstract Book

S319

ESTRO 2021

Conclusion The FISTArcPT algorithm proposed for Arc Proton Therapy treatment plan optimization has successfully solved the energy layer selection problem by generating acceptable plans with good dosimetry results similar to IMPT plans. The definition of our objective function will be refined with an energy sequencing term that favors small consecutive decrements of energy to further reduce the irradiation time. OC-0422 Proton-based SBRT of tumors in the liver: Prescription strategy, motion and interplay effects E. Worm 1 , R. Hansen 1 , M. Høyer 2 , B. Weber 2 , H. Mortensen 2 , P.R. Poulsen 3 1 Aarhus University Hospital, Dept. of Oncology, Aarhus, Denmark; 2 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark; 3 Aarhus University Hospital, Dept. of Clinical Medicine, Aarhus, Denmark Purpose or Objective To optimize the trade-off between target and liver dose, a non-uniform dose prescription is often used in x- ray-based liver SBRT. However, the use of non-uniform dose lacks investigation in pencil beam scanned (PBS) proton SBRT and PBS-proton SBRT is prone to interplay effects between breathing and proton beam motion. This study investigated non-uniform and uniform dose prescription in proton liver SBRT, including the effects of target motion during planning 4DCT and treatment delivery. Materials and Methods The study was based on 42 previous x-ray SBRT fractions delivered to 14 patients using electromagnetic motion-monitoring (Calypso system). First, two intensity-modulated proton plans both with a prescribed CTV mean dose of 48 Gy in three fractions were constructed for each patient (Fig.1A2-3). The plans were optimized (Varian Eclipse) to cover the CTV with 95% of the prescribed dose in static nominal plan and to cover the CTV robustly with either 95% (uniform dose) or 67% (non-uniform dose) of the prescribed dose. The robust optimization included 3.5% range uncertainty and the 3D motion observed in the planning 4DCT. Secondly, the uniform plan (with a wider high-dose area) was downscaled to be isotoxic with the non-uniform plan using a NTCP model for radiation induced liver disease (Dawson et al., IJROBP 2002, Fig.1A5). Thirdly, an in-house treatment simulator was used to simulate delivery of both plans for a range of scenarios based on 4DCT or Calypso motion, with and without a spot-adapted repainting scheme with the spot repaintings spread over the full breathing cycle (Fig.1A6-8, Fig. 1B). The delivered CTV dose was calculated in the TPS by modelling tumour motion as proton spot shifts and in-depth motion as spot energy changes. Results The mean (±SD) cranio-caudal motion during 4DCT and treatment was 9.4±3.8mm and 17.9±7.4mm (with baseline-shifts), respectively. The CTV dose deterioration depended highly on the motion scenario (4DCT or Calypso), number of fractions and use of repainting; see examples in Fig.2A. Although the uniform plan was more motion robust it resulted in lowest CTV dose due to the isotoxicity normalization. With Calypso motion during three fractions (scenario 5, no re-painting), the achieved CTV dose was on average 24.8±8.4% (D mean ) and 16.2±11.3% (D 98 ) higher with non-uniform plans than with uniform plans. The interplay-induced increase in D 2% relative to the static plans was reduced from 3.2±4.1% without repainting to -0.5±1.7% with repainting for non-uniform plans and from 1.5±2.0% to 0.1±1.3% for uniform plans. Fig 2B-C summarize the delivered CTV D98% and D2% for all motion scenarios. Conclusion Non-uniform dose prescription in proton SBRT may provide higher tumor doses than uniform prescription for the same complication risk. Due to motion variability, target doses estimated from 4DCT motion may not accurately reflect the delivered dose. Considerable interplay effects were mitigated by fractionation and spot-adapted repainting.