ESTRO 36 Abstract Book

S158 ESTRO 36 _______________________________________________________________________________________________

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.

Conclusion Motion induced errors in dose were accurately and continuously reported by gamma evaluations within two seconds of occurring. Such monitoring may improve patient safety by treatment intervention in case of gross treatment errors and may help to expedite clinical use of tracking. While developed mainly with tumour tracking in mind its use is also readily available for standard non- tracking treatments. OC-0305 Validation of Dynamic Treatment-Couch Tracking for Prostate SBRT S. Ehrbar 1 , S. Schmid 1 , S. Klöck 1 , M. Guckenberger 1 , O. Riesterer 1 , S. Tanadini-Lang 1 1 University Hospital Zürich, Department of Radiation Oncology, Zurich, Switzerland Purpose or Objective In stereotactic body radiation therapy (SBRT) of prostatic cancer, a high dose per fraction is applied to the treated region with steep dose gradients. Intrafractional prostate motion can occur unpredictably during the treatment and lead to target miss. Missing the target results in high doses to nearby organs which can cause complications. It is essential for a prostate SBRT treatment to observe and mitigate this motion. Dynamic treatment-couch tracking is a real-time adaptive therapy technique, compensating the prostate displacement by counter-movement with the treatment couch. This work investigated the dosimetric benefit of couch tracking for prostate SBRT treatments in the presence of prostatic motion. Material and Methods Ten previously treated prostate cancer patients with one index lesion were selected. Treatment target volumes (prostate and index lesion), and organs at risk (OAR: bladder, rectum and urethra) were delineated using the patient’s treatment CT and MRI scans. SBRT treatment plans with integrated boost were prepared with a prescribed dose of 5x7 Gy to the prostate and 5x8 Gy to the index lesion. The treatment plans were applied with a linear accelerator to a phantom, which was either i) in static position, ii) moved according to five prostate motion curves without motion compensation or iii) with real-time compensation using electromagnetic guided couch tracking. Electromagnetic transponders were mounted on the phantom surface and their geometrical position was evaluated in the tracked and untracked situation.

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,