ESTRO 2023 - Abstract Book

S74

Saturday 13 May

ESTRO 2023

rkB ( S ) (Fig. (a)) varies from 0.93 to 1.04 for S =0.5 cm and strongly depends on detector type and orientation. Uncertainties on OF were estimated by accounting for the statistical uncertainties of the Monte Carlo data, fitting and experimental uncertainties. Fig. (b) demonstrates that all differences ( Δ ) of OF ( S ) from their average (per field ) are consistent with their associated uncertainties (< 2.2 %) for a coverage factor k =2 and for most data points on the level k =1. Comparing the OF ( S ) calculated using the TRS-483 formalism (i.e. ignoring rkB ( S )) yields Δ values of -5.4% to +5.2% (min-max). Conclusion A new method to extend TRS-483 for small field dosimetry in MR-linacs, based on existing output correction factor data sets for B =0 T, was developed and validated. Output correction factors in presence of magnetic field strongly depend on detector type and orientation. Despite these differences and in contrast to the existing TRS-483 formalism, the proposed method and calculated output correction factor data yields consistent OF ( S ) results within their uncertainty (< 2.2%), which is on the same level as reported in TRS-483 for conventional linacs. [1] Casar, B, et al, Med. Phys. 2020 OC-0114 Patient specific quality assurance for synthetic CT in MR-only radiotherapy of the abdomen R. Dal Bello 1 , M. Lapaeva 1 , A. Agustina 1 , P. Wallimann 1 , M. Günther 2 , E. Konukoglu 3 , N. Andratschke 1 , M. Guckenberger 1 , S. Tanadini-Lang 1 1 University Hospital Zurich, Department of Radiation Oncology, Zurich, Switzerland; 2 University of Zurich, Department of Informatics, Zurich, Switzerland; 3 ETH Zurich, Computer Vision Laboratory, Zurich, Switzerland Purpose or Objective The integration of artificial intelligence will play a fundamental role for further developing radiotherapy in the upcoming years. However, it requires the implementation of dedicated quality assurance (QA). This applies in particular in the context of MR-only radiotherapy, where the simulation CT required for dose calculation is substituted by a synthetic CT (sCT). MR- only radiotherapy simplifies the treatment planning workflow, but poses new challenges for patient specific QA (PSQA). Materials and Methods This study retrospectively analysed 20 patients treated at a hybrid MR-Linac for abdominal lesions to assess different PSQA techniques for sCT. The patients were selected to equally cover four specific subgroups presenting the following features along the beam path: (a) standard cases, (b) air pockets, (c) lungs and (d) implants. Each patient underwent an MR simulation at the MR-Linac (sequence TrueFISP, field 0.35T) followed by a CT. The retrospective simulation of an MR-only workflow assumed that the electron density map for dose calculation was computed from a sCT generated using a neural network (CycleGAN) from the MR data. Figure 1 displays the four methods investigated to generate an independent electron density map for PSQA: (i) water override of the body mask, (ii) manual bulk override of five tissue classes, (iii) sCT from an independent neural network (NN) based on pix2pix model and (iv) deformed CT from the CT simulation (dCT). IMRT plans were recalculated with preset MU for the four PSQA approaches and DVH parameters were extracted. The statistical significance was assessed with a non-equivalence test for dependent paired samples (TOST-P).

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