ESTRO 37 Abstract book

S961

ESTRO 37

anatomical details of real patients can be obtained using an image acquired with the photons emitted from a HDR 192 Ir source. A clinical trial for prostate and gynecological patients is in preparation. EP-1789 Sensitivity analysis of EPID-based 3D dose reconstruction for VMAT QA A. Alhazmi 1 , C. Gianoli 1 , S. Neppl 2 , J. Martins 1 , S. Veloza 1 , M. Podesta 3 , F. Verhaegen 3 , M. Reiner 2 , C. Belka 2 , K. Parodi 1 1 Ludwig-Maximilians-Universität München, Department of Medical Physics- Faculty of Physics, Garching, Germany 2 LMU Munich, Department of Radiation Oncology, Munich, Germany 3 Maastricht University Medical Center MUMC, Department of Radiation Oncology MAASTRO, Maastricht, The Netherlands Purpose or Objective The goal of this study is to investigate the sensitivity of an EPID-based 3D dose reconstruction algorithm to detect geometric and dosimetric errors of VMAT plans for A VMAT plan for head & neck case was generated using the TPS Monaco®. The plan was delivered using ELEKTA synergy® linac equipped with PerkinElmer® aSi EPID. The acquired frame images were used to reconstruct 3D dose distribution with 1 mm isotropic resolution using an in- house developed algorithm. The algorithm converted the acquired frame images into planar dose distribution and back-projected them upstream into a numerically modeled cylindrical water phantom. The planar dose distributions of the water phantom were deconvolved by depth-specific scatter and attenuation kernels, thus reconstructing a 3D dose distribution. The kernels were obtained by making use of scatter and attenuation models to iteratively estimate the parameters from a set of reference measurements. The obtained parameters served as a look-up table for reconstruction of arbitrary measurements. For each frame image, the reconstructed 3D dose distribution was rotated in accordance with the correlated projection angle of the gantry. A sum of the reconstructed 3D dose distributions resulted in the integrated 3D dose distribution of the VMAT delivery. For geometric sensitivity testing, first, two selected neighboring leaves were shifted by 2 mm in all control points of the original plan file. Second, a controlled rotational shift of the gantry angles in the plan files of 1 degree was applied. For dosimetric sensitivity testing, the total monitor units of the original plans were increased by 4%. The resulting 3D dose distributions were compared to the original plan using the well-known gamma evaluation with (3%, 3 mm) as acceptance criteria. Results Figure (1) shows different views of the original 3D dose distributions calculated by the TPS and the reconstructed 3D dose distribution. The gamma evaluation between the plan and the unmodified acquisition was fulfilled by exceeding the 95% of the acceptance criteria. The modified acquisitions resulted in decreased passing rates of 93%, 91% and 83% of the acceptance criteria for the shifted leaves, gantry angle and added dose, respectively. Figure (2) (a, c and d) show the gamma evaluations of the unmodified and modified acquisitions at the iso-center plane. In Figure (2) (b) the effects of the leaves shift at the corresponding slice is illustrated. Quality Assurance (QA). Material and Methods

Conclusion The gamma evaluation was able to detect both geometric and dosimetric errors. Thus, the in-house developed EPID-based 3D dose reconstruction algorithm shows adequate sensitivity to errors for plan specific pre- treatment QA of advanced VMAT delivery. Moreover, the gamma evaluation could support the interpretation of the error causes. EP-1790 Patient specific quality assurance on the MRI Linac for the First-in-Man study B. Van Asselen 1 , S.L. Hackett 1 , J.W.H. Wolthaus 1 , J.H.W. De Vries 1 , I.M. Jürgenliemk-Schulz 1 , J.J.W. Lagendijk 1 , B.W. Raaymakers 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands Purpose or Objective This work describes the tests for patient-specific quality assurance (QA) performed during the clinical introduction of the MRI linac (Elekta AB, Sweden) and used for the