ESTRO 2021 Abstract Book

S1675

ESTRO 2021

Conclusion The reduction of the PTV margins with a POD concept, based on daily image analysis, showed a statistically significant NTCP reduction for both rectum and bowel bag, proving the potential toxicity reduction. POD will be implemented clinically for bladder cancer patients at our center. PO-1969 The contribution of on-board hybrid MRI for planning optimization: movement analysis of the uterus C. Votta 1 , L. Boldrini 1 , B. Merola 1 , G. Chiloiro 1 , A. Romano 1 , D. Cusumano 1 , G. Turco 1 , M. Bianchi 1 , P. Cornacchione 1 , L. Indovina 1 , M.A. Gambacorta 1 , V. Valentini 1 1 Fondazione Policlinico Universitario A. Gemelli IRCCS, Diagnostica per Immagini, Radioterapia Oncologica ed Ematologia, Rome, Italy Purpose or Objective Exclusive chemoradiotherapy (CRT) is the main treatment option for locally advanced cervical cancer (LACC) and due to its anatomical site, organ motion (OM) and volumetric changes of the target represent a significant issue during radiotherapy (RT) delivery. Magnetic Resonance-guided Radiotherapy (MRgRT) could represent an improvement to current standard, thanks to the higher contrast of Magnetic Resonance Imaging (MRI) and to the possibility to deliver treatment plans based on the anatomy of the day (online adaptive). Aim of this study is to analyse the volumetric changes of the CTV represented by the cervical-uterine structure (CUS) during RT treatment. Treatment plan adaptation strategies are also proposed in order to take full advantage of the MRgRT delivery technology and successfully manage OM. Materials and Methods Ten patients affected by LACC undergoing CRT were retrospectively analysed. Each patient received 22-25 RT daily fractions using the MRIdian system (ViewRay, Mountain View, California, US), according to the adopted clinical protocols (tumor BED1o 44.2-51.9 Gy). All daily positioning 0.35 T MR-scans were registered with the planning MR-scan by rigidly aligning the bony anatomy. The CUS were delineated and the inter-fraction volume changes were assessed in each MR scan (figure 1). Interfractional displacements of the CUS were evaluated by comparing its geometric centroid (GC) of each daily fraction with the ones of the coregistered planning scan (∆GC-Plan) and of the previous fraction (∆GC-Fx). Different PTVs were obtained adding different isotropic margins to CUS of the planning scan (from 5mm to 15mm, with a 1mm step). The margins including 95% of the CUS during the whole treatment in 95% of pts were considered clinically adequate.

Results A total of 232 pre-treatment MR scans were reviewed and contoured. Figure 2 shows the volumetric changes of the CUS in relation to increasing dose. The median volume reduction was 32.4% (53.6% - 26.2%) . A significant displacement between the CUS of every single fraction and the planning position was found (∆GC-Plan mean = 0.93±0.21 cm), mostly located in the caudo-cranial (0.48±0.17 cm) and posterior-anterior (0.53±0.13 cm) direction. Analyzing the CUS position in relation to the previous fraction, we noted larger mobility in the first 5 fractions (ΔGC-Fx mean= 0.96±0.09 cm).