ESTRO 36 Abstract Book

S157 ESTRO 36 2017 _______________________________________________________________________________________________

Radiation dose was measured within the phantom by a biplanar diode array. The dosimetric performance of couch tracking was compared to no compensation: Gamma agreement and other dose parameters were evaluated within the biplanar array, as well as target- and organ- specifically. Results The root-mean-square error of the motion traces (range: 0.8-4.4 mm) was substantially reduced with couch tracking (0.2-0.4 mm). Spikes (>1 mm) in the compensated motion curve were only observed at steep gradients (>7.5 mm/s). The dose measurements with the phantom showed on the 1%/1 mm level significantly better gamma agreement with tracked motion (range: 83.4%-100%) than with untracked motion (28.9%-99.7%). Also with the 2%/2 mm criterion, gamma agreement was significantly superior for the tracked motion (98.4%-100%) compared to the untracked (52.3%-100%) (see Fig. 1). Also the organ specific evaluation resulted in significantly better target coverage with tracking, however the dose to the rectum and bladder showed a dependency on the motion direction.

simulations were performed for spot or raster scanning (positioning with/without beam pulse between successive spots), for constant and varied beam current, and for cyclotron or synchrotron beam dynamics (e.g. continuous or pulsed beam). The resulting 4D plans were compared using dose-volume metrics (D5-D95, V95) for the CTV, as well as total predicted treatment time (TT).

Results Independent of the delivery scenario and prescribed dose, neither gating alone nor rescanning alone could mitigate motion effects completely, with residual interplay effects (D 5-95 ) of more than 10-20% being observed for GW=5mm w.o. rescanning (shown by purple error-bars). Moreover, the D 5-95 of synchrotron based simulations were found to be >5% higher than for cyclotron scenarios. However, with re-gating (re-scanning + gating, shown by green and blue error-bars), motion mitigation performance was found to be similar effective (close to static reference) for all scanning dynamics and rescan modes, with the main difference being only in treatment efficiency. Without any mitigation, mean TT’s for the 2Gy/12Gy plans were 2x/3x longer for synchrotron than for cyclotron scenarios. For re- gating (GW5+LS5), mean TT’s of synchrotron based plans were on average 2.5x higher when combined with LS and 3.5x higher when combined with VS. Moreover, the advantage of varying beam current has been demonstrated by the approximately constant TT as a function of prescription dose. In addition, for the high dose scenario, variations caused by differences in geometry, motion amplitude, field direction and starting phase, are smaller for varying beam current scenarios in comparison to corresponding constant scenarios.

Conclusion Couch tracking was able to mitigate the prostate motion and improved the dosimetric accuracy of prostate SBRT. The treatment couch was able to compensate the prostatic motion with only some minor residual motion at steep motion gradients. Therefore, couch tracking combined with electromagnetic position monitoring for prostate SBRT is feasible and improves the accuracy in treatment delivery when prostate motion is present. OC-0306 Is re-gating a robust motion mitigation approach independent of PBS scanning scenario? Y. Zhang 1 , I. Huth 2 , M. Wegner 2 , D.C. Weber 1 , A.J. Lomax 1 1 Paul Scherrer Institute PSI, Center for Proton Therapy, Villigen PSI, Switzerland 2 Varian Medical Systems, Particle Therapy GmbH, Troisdorf, Germany Purpose or Objective Different scanned proton therapy (PBS) systems provide different scanning dynamics, directly changing the temporal interference between pencil beam delivery and tumour motion. With this study, we have systematically evaluated interplay effects, and compared motion mitigation performance, for different PBS scanning delivery scenarios. Material and Methods Using 6 4DCT(MRI) datasets of liver tumours, with irregular motions >10mm (CTV: 100-400cc; period: 5.3/6.3s), 4D treatments assuming different prescription doses (2/12Gy), field directions (AP/LR) and mitigation approaches (3 gating windows (GW) with or without 1-9 layered (LS) or volumetric (VS) rescanning) were simulated for 8 PBS scanning scenarios (see table). As such,