ESTRO 36 Abstract Book

S156 ESTRO 36 2017 _______________________________________________________________________________________________

OC-0304 Real-time gamma evaluations of motion induced dose errors as QA of liver SBRT tumour tracking T. Ravkilde 1 , S. Skouboe 1 , R. Hansen 1 , E.S. Worm 1 , P.R. Poulse n 1 1 Aarhus University Hospital, Department of Oncology, Aarhus N, Denmark Purpose or Objective Organ motion during radiotherapy can lead to serious deterioration of the intended dose distribution. As modern radiotherapy shifts increasingly towards escalated doses, steeper dose gradients and hypofractionation, the demands on accurate delivery increase concurrently. A large body of studies show that tumour tracking can be applied to mitigate the effects of motion and restore dose fidelity, yet clinical introduction seems reluctant. In this study we report on a method for continuous evaluation of the tracking dose delivery that conforms to common dose analysis practice and can be acted upon in real time. Material and Methods Experiments were performed on a TrueBeam linear accelerator (Varian Medical Systems) with target motion being recorded by an electromagnetic transponder system (Calypso, Varian Medical Systems). A HexaMotion motion stage (Scandidos) reproduced the liver motion traces for five different liver SBRT patients as previously measured using intrafraction kV imaging. VMAT SBRT treatment plans were delivered to the moving phantom with MLC tracking, without tracking (simulating the actual delivery) as well as to a static phantom for reference (planned delivery). Temporally resolved dose distributions were measured at 72 Hz using a Delta4 dosimeter (Scandidos). Accelerator parameters (monitor units, gantry angle, MLC leaf positions, etc.) were streamed at 21 Hz to prototype software that performed continuous reconstruction of the dose in real time by a simplified non-voxel based 4D pencil beam convolution algorithm. Also in real time, but on a separate thread, 3%/3mm gamma evaluations were calculated continuously throughout beam delivery to quantify the deviation from the planned intent. After experiments, the time-resolved gamma tests were compared with the same quantities from the measured data. Results The motion induced gamma errors were well reconstructed both spatially (Figure 1) and temporally (Figure 2). In 95% of the time both actual and planned doses were reconstructed within 100 ms. The median time for reconstruction was 65 ms, which translates into a typical frequency of about 15 Hz. Asynchronously, but also continuously, 95% of gamma evaluations were performed within 1.5 s with the median being at 1.2 s. Over all experiments the root-mean-square difference between reconstructed and measured gamma failure rates was 2.9%.

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.