ESTRO 2021 Abstract Book

S1442

ESTRO 2021

Figure 1. Irradiated radiochromic films. Left: a clear misalignment can be seen for patient 5. Right: Better alignment can be appreciated for patient 11.

Results The postprocessing analysis of the dose distribution measured on the films provides a quantitative estimate of the misalignment between the collimator and the disk. For the 11 analyzed patients the average surface outside the protective disc is 15% of the irradiated area (range 0% to 46%) corresponding to an average surface of 3.4 cm2 (range 0 to 9.8 cm2) and in agreement with the scarce results reported in the bibliography. No relationship was found between any of the analyzed parameters and the percentage of irradiation outside the field (Table 1). Table 1. Patient characteristics, treatment parameters and disc alignment evaluation.

Conclusion Our preliminary results show a considerable amount of misalignment between the applicator and the disk. In- vivo verification should be routinely used as a quality check during IOERT and future methods to improve the procedure should be tested and implemented.

PO-1718 Variations in MR linac beam performance – daily EPID-based results from 22 months of clinical use A. Bertelsen 1 , E.L. Lorenzen 1,2 , K.L. Gottlieb 1 , S.N. Agergaard 1 , R.L. Christiansen 1 , H.R. Jensen 1 , H.L. Riis 1,2 , C. Brink 1,2 , U. Bernchou 1,2 1 Odense University Hospital, Laboratory of Radiation Physics, Department of Oncology, Odense, Denmark; 2 University of Southern Denmark, Department of Clinical Research, Odense, Denmark Purpose or Objective An MR linac is a complex system, which demands fast, adequate, and efficient daily QA. The Electronic Portal Image Device (EPID) mounted on the gantry of an 1.5 Tesla MR linac can be used for part of this QA. The current study reports on the variations in daily output, beam energy, and beam geometry measured on the EPID, such that tolerances for future daily EPID based QA can be established. Materials and Methods The on-board gantry-mounted EPID was used for daily recording of a static beam (100 MU) with an 80 mm X 160 mm rectangular field within the EPID field of view. The beam had a fixed gantry angle at 177.8 degrees such that a cryostat component fixed in space was visible in the EPID image. Using the record and verify (R&V) system, this QA beam has been delivered daily since May 2019. Besides the above mentioned QA purpose, the QA beam was also used to check the communication between the linac and the R&V system. The recorded EPID images were exported to a DICOM server, and automatically analyzed using an in-house developed MATLAB program. The program extracted information about output, beam flatness and symmetry, jaw position, gantry angle and MLC leaf positions (those visible in the field of view - 22 MLC leaf pairs). A steepest gradient image analysis was used to determine the MLC, jaw, and gantry positions within the images. Flatness and symmetry were accessed by comparison of image intensities in central and lateral parts of the image. The output information was obtained from the overall scaling of the image. Data was corrected for changes in