ESTRO 36 Abstract Book

S157 ESTRO 36 _______________________________________________________________________________________________

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%.

weeks, keeping the same positioning. The intra-fraction reproducibility of the lung anatomy during breath hold was investigated, by comparing the MRI of the first breath hold with the three other MRIs of the same session. The inter- fraction anatomical reproducibility was investigated by comparing the first breath hold MRI of the first session with the four MRIs during the second session. To avoid any influence of setup variation, first a global rigid image registration was performed. Then the lung volume was semi-automatically segmented to define a region of interest for the deformable image registration (DIR). DIR was performed using Mirada RTx v1.2 (Mirada Medical, Ltd.), with a DIR grid resolution of 3.5x2x3 mm 3 . The deformation vector fields were analyzed using MATLAB v2014b. Magnitudes of the deformation vectors were calculated and combined for all five volunteers. The lung volumes were divided into six segments, to analyze the anatomical displacements on a local level. A boxplot showing the intra- and inter-fraction displacements with a schematic view of the six segments can be seen in figure 1. Results The lung volumes for all breath holds varied by 2% within and 7% between fractions. Looking at all five volunteers, up to 2 mm median intra- and inter-fraction displacements were found for all lung segments. The anatomical reproducibility decreased towards the caudal regions. Inter-fraction displacements were larger than intra- fractional displacements. Maximum displacements (99.3% of the magnitude vectors) reached 6 mm intra-fractionally and did not exceed 8 mm inter-fractionally.

Conclusion While the lung volume differences were insignificant, relevant anatomical displacements were found. Moreover, a trend of increased displacements over time could be seen. ABC mitigates motion to some extent. Nevertheless, the remaining reproducibility uncertainties need to be considered during scanned proton therapy treatments. As next step, we aim to include this knowledge in a model to estimate their dosimetric influence for scanning proton therapy.