ESTRO 2021 Abstract Book

S1393

ESTRO 2021

routine and 5 with IMR3 routine.Significative low contrast detectability improvements are observed for 4D reconstruction (phase binning, bin width 10%) with Soft IMR level 2 or 3 image definition soft tissue, with 1% contrast now visible above CTDI of 14mGy. HU are not influenced by IMR level or image definition type and HU to mass density relationship is nearly identical to iDose which facilitates the clinical adoption for dose calculation.

Conclusion For a given dose level, the knowledge-based reconstruction algorithm IMR can improve both spatial resolution and low contrast detectability in comparison to iDose, improving overall image quality. IMR advantages relative to iDose increase when decreasing slice thickness and exposure. IMR 2 or 3 Tissue image definition significantly improve low contrast detectability for 4DCT acquisitions, parameter of interest in the abdomen area. HU numbers do not depend on image definition and noise reduction level and are similar to iDose. PO-1670 Clinical implementation of MRI-only radiotherapy workflow for prostate cancer with a standard linac M. Felisi 1 , A.F. Monti 1 , D. Lizio 1 , S. Nici 1 , S. Riga 1 , R.G. Pellegrini 2 , B. Bortolato 3 , M.G. Brambilla 1 , C. Carbonini 1 , C. Carsana 3 , D. Sibio 3 , C. Potente 3 , A. Vanzulli 4 , M.F. Palazzi 3 , A. Torresin 1 1 ASST GOM Niguarda, Medical Physics Department, Milan, Italy; 2 Elekta AB, Department of Radiation Oncology, Stockholm, Sweden; 3 ASST GOM Niguarda, Radiotherapy Department, Milan, Italy; 4 ASST GOM Niguarda, Radiology Department, Milan, Italy Purpose or Objective The purpose of this work is the clinical implementation of radiotherapy treatment workflow for prostate cancer based entirely on MR-images (MRI-only) with a standard linac, using the technological tools available in a modern general hospital. The main concern of this approach is that MRI-voxels are related to tissue proton density, so to properly calculate the dose distribution in a commercial TPS, it is necessary to convert all voxels in term of electron density (ED) by creating a synthetic CT (sCT). Materials and Methods To create sCTs, a hybrid method that is a combination of bulk ED and multi-atlas-based approach, is proposed. MR images from 20 patients were acquired by using an optimized 3D T1 VIBE Dixon sequence with a large FOV 1.5 T Siemens Aera MRI-scanner, flat table couch and fixation devices for feet and knees. Although the scanner is equipped with active shimming, geometric distortions were assessed. Contours of the patients were delineated by identifying bone structures and main organs-at-risk (OARs), to create atlases as reference database for an auto-segmentation software. Finally, sCTs were generated in the TPS, starting from the auto-segmented MR-images (Figure 1) by assigning to each structure a bulk ED, averaged over 20 CTs of patients who underwent to prostate radiotherapy in our center.

Twenty 10MV-photon VMAT prostate plans were optimized on planning CT and recalculated on the corresponding sCT. Dosimetric differences were evaluated by using quantitative methods such as dose-volume histogram (DVH), as shown in Figure 2, and 2D-local gamma analysis.