ESTRO 2023 - Abstract Book

S827

Monday 15 May 2023

ESTRO 2023

1 Inselspital, University Hospital Bern and University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland; 2 Inselspital, Bern University Hospital and University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland; 3 Inselspital, Bern University Hospital and University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland Purpose or Objective Electron beam radiotherapy for tumor bed boost irradiation after whole breast irradiation is currently performed using applicators and cut-outs mounted on a C-arm linear accelerator. Replacing the applicators and cut-outs with the already installed photon multileaf collimator (pMLC) reduces the effort for electron therapy as no cut-out needs to be fabricated and no applicator has to be mounted. Furthermore, the pMLC facilitates modulated electron radiotherapy (MERT). Electron modulated arc therapy (EMAT) extends MERT by dynamic gantry rotation and dynamic pMLC movement during beam on with the potential to reduce delivery time. The aim of this work is to develop a treatment planning process for pMLC based EMAT for breast boost irradiation. Materials and Methods To create EMAT plans, a gantry range is defined for all available electron beam energies. The electron arcs maintain a shortened source-surface distance (SSD) of 80 cm by synchronized table movement to reduce air scatter. A fluence map optimization is performed for the electron arcs and the arcs are ranked based on their relative contribution to the PTV mean dose. Next, a direct aperture optimization is performed with the two highest-ranked electron arcs, resulting in a two-arc EMAT plan. EMAT plans are created retrospectively for three clinically motivated cases: two left and one right breast boost. The treatment plan quality and estimated delivery time of the EMAT plans are compared to cut- out plans using a single static electron field and to retrospectively created MERT plans using one beam direction with an SSD of 80 cm. Results The dose homogeneity index ( (D2% - D98%)/D50% ) is on average improved by 2% when comparing the EMAT to the MERT plans and improved by 18% when comparing the EMAT to the cut-out plans. The mean dose to the ipsilateral lung is on average 2% lower for the EMAT plans compared to the MERT plans, but 3% higher compared to the cut-out plans. The mean dose to the heart is lower than 0.5% of the prescribed dose for all plans of all considered treatment techniques. The estimated delivery time is on average 1.1 min for the EMAT plans and 2.7 min for the MERT plans. Conclusion A treatment planning process for EMAT applied to breast boost irradiation was developed. Improvements in the delivery time and planning of EMAT over MERT further facilitate more efficient electron treatments. This work was supported by grant 200021_185366 of the Swiss National Science Foundation and Varian Medical Systems. PD-0979 Evaluation of commercial deep learning MRI reconstruction for synthetic CT generation in prostate RT C. Jamtheim Gustafsson 1,2 , J. Scherman 1 , A. Gunnlaugsson 1 , L.E. Olsson 3 1 Skåne University Hospital, Dept Haematology, Oncology and Radiation Physics, Lund, Sweden; 2 Medical Radiation Physics, Lund University, Department of Translational Sciences, Malmö, Sweden; 3 Medical Radiation Physics, Lund University , Department of Translational Sciences Malmö, Malmö, Sweden Purpose or Objective The use of deep learning (DL) for magnetic resonance image (MRI) reconstruction has been implemented by several commercial vendors and allows for both image noise- and scan time reduction. This can provide several benefits when using MRI for radiotherapy planning purposes. The aim of this work was 1) to evaluate the compatibility between a commercial DL MRI reconstruction product and a commercial synthetic CT (sCT) generation software and 2) to quantitatively assess the Hounsfield (HU) and dosimetric integrity of sCT created from such MR images. Materials and Methods Twenty-four prostate cancer patients were prescribed ultra hypofractionated radiation therapy (RT) with 42.7 Gy, 7 fractions, in a clinical MRI-only treatment workflow using a General Electric (GE, Chicago, USA) 3T Architect MRI system together with a lightweight AIR receiver coil. Within the clinical MRI acquisition protocol, a large field of view (LFOV) T2 weighted MRI image volume was acquired for sCT conversion (sCT_orig) using Spectronic MRI planner v.2.4.14 (Spectronic Medical AB, Helsingborg, Sweden). This sCT generation has previously been validated against CT. In parallel, the MRI raw data from the LFOV acquisition was used in DL MRI reconstruction using the GE Air Recon DL product for host version MR29.1 and a new MRI image volume was created together with a corresponding sCT (sCT_DL). To assess differences in HU values between sCT_orig and sCT_DL, HU differences outside the patient, in fat, muscle, spongy bone and compact bone was analyzed. To assess RT dose differences in target and organs at risk, clinical RT structures except the body structure were copied from the sCT_orig to the sCT_DL, clinical treatment plan was transferred with the same number of monitor units and dose was recalculated on sCT_DL. The created sCT_DL was visually inspected and the body volume for all sCT were calculated. Results HU differences outside the patient, in fat, muscle, spongy bone and compact bone had a mean absolute error (± 1 STD) in the cohort of 1.5±0.4, 1.5±0.3, 1.2±0.4, 4.9±1.1 and 7.4±2.2 HU (n=24), respectively. The mean dose differences for body, femoral heads, bladder, rectum, CTV and PTV were all positive and below 0.06 Gy (Fig.1, 0.1% difference, n=23). The sCT_DLs had a visually excepted appearance and the sCT body volume were larger for all patients but three compared to sCT_orig, with a median cohort difference of 15 cm3 (spread in cohort body volume was 11603-21542 cm3).