ESTRO 36 Abstract Book

S956 ESTRO 36 _______________________________________________________________________________________________

2 Varian Medical Systems, Reserach and Development, Palo Alto- CA, USA Purpose or Objective To study the performance, image quality and clinical potential of a novel fast MV-CBCT panel mounted on a straight-through linac with 6X-FFF (filter-free) beam. Material and Methods A prototype MV imager is mounted in-line with the linac at 154 cm source-to-imager distance. MV-CBCT imaging is performed with 28 cm fixed width (projected at isocenter- 100 cm) and variable field length (up to 26 cm max) FOV, 200º arc, 15 sec rotation time, at 5MU and 10MU dose levels. Images of a Catphan® 604 phantom are analyzed in terms of noise, uniformity, spatial and contrast resolution, and contrast linearity. The results are compare with those from Truebeam kV-CBCT imager, and potentially, a Tomotherapy MV-CT imager, all benchmarked to diagnostic CT scanner images. Imaging dose will also be assessed for completeness of testing of the MV-CBCT The uniformity and noise of the MV-CBCT was acceptable but not as good relative to diagnostic CT and kV-CBCT, with variation of 32HU from the center to the periphery. Spatial resolution is shown in Fig. 1 with 3lp/mm for 5MU and 4lp/mm for 10MU dose levels, as compared to 7lp/mm for both kV-CBCT and diagnostic CT. The low contrast resolution of the MV-CBCT was >> 1%, compared to 1.0% and 0.5% for kV-CBCT and diagnostic CT. Contrast linearity and sensitometry is shown in Fig. 2, with the MV-CBCT (and 10MU dose level) being the modality that deviates especially for higher density objects like Acrylic, Delrin and Teflon (green data points). Additional tests with varying field length (small, medium and max length) for the MV-CBCT did not show any correlation and/or improvement to the image quality results. Imaging dose will be assessed and presented based on absolute beam output calibration with a 6cc ion chamber and OSLDs. The Tomotherapy data and analysis is underway for comparisons. system. Results

For each dynamic beam delivery the TrueBeam system also produces a single log of the main system parameters in a trajectory log text file. The TrueBeam Log Viewer application can list these parameters versus time for user- selected accelerators and data interval. All lists can be sorted with respect to any column, non- clinical events can be ignored, events not matching a user- selected main fault code or fault description string can be ignored, and all lists can be exported to Excel for further analysis.

Results Sorting the list of events with respect to the main fault code provides a fast overview of the number faults for each fault type and accelerator, facilitating an easy prioritization of faults (see figure 1). In addition, treatment plan details and mechanical axes for each event (for example beam energy, the number of monitor units, the patient ID, and the gantry angle) are valuable for the identification of the root cause of the corresponding fault. A more detailed insight into the root cause of a fault can be gained by analyzing node records of the corresponding event in Excel. An example of a node record for a gantry fault is shown in figure 2. Also trending of the main system parameters from the trajectory log text files with Excel is a strong troubleshooting tool.

Conclusion The TrueBeam Log Viewer application is an efficient tool both for obtaining an overview of existing faults and for identification of the root cause of faults. In our clinic the application has provided a much more substantiated prioritization of service tasks and faster identification of the root cause of faults. Varian has no service tools with this capability. EP-1738 Performance of a new EPID panel and opportunities for a fast MV-CBCT acquisition. C. Kennedy 1 , C. Ling 2 , R. Scheuermann 1 , D. Mihailidis 1 , J. Metz 1 1 University of Pennsylvania, Radiation Oncology, Philadelphia, USA