ESTRO 2022 - Abstract Book

S1166

Abstract book

ESTRO 2022

Fig 2. A & C: per-patient average rectal V90% and bladder mean dose from daily CBCTs (± 1 SD) compared to the planning CT. B & D: per-study relationship (n=339) between volume difference and rectal V90% difference (B) / bladder mean dose difference (D). Conclusion Rectum and bladder volume variations were found to be substantial for all patients. The rectal V90% variation was found to be high independently of the rectum volume variation. This could indicate that not only the rectal volume but also its position possibly plays a role in how much dose is delivered to the rectum at each fraction. This could in return advocate for considering daily matching on CBCT images, especially in cases where the target is localized directly on the anterior wall of the rectum.

PO-1374 Feasibility of magnetic resonance-guided adaptive post-prostatectomy radiotherapy

S. Hassan 1 , M. Jameson 1,2 , V. Batumalai 1,2 , D. Crawford 1 , Z. Moutrie 1 , L. Hogan 1 , C. Loo 1 , M. Picton 1 , C. Pagulayan 3 , U. Jelen 3 , S. Alvares 1 , M. Heinke 3 , S. Sampaio 1 , K. Simon 1 , T. Twentyman 1 , N. Dwivedi 4 , J. de Leon 1 1 GenesisCare St Vincent's Sydney, Radiation Oncology, Sydney, Australia; 2 University of New South Wales, St Vincent's Clinical School, Sydney, Australia; 3 GenesisCare St Vincent's Sydney, Radiation Oncology, Sydney, Australia; 4 MIM Software Inc , Clinical Support Engineering , Cleveland , USA Purpose or Objective Radiotherapy is a mainstay of treatment in locally recurrent prostate cancer following prostatectomy. However, target volumes are highly variable and anisotropic margins are often utilised with traditional linear accelerator treatment to account for inter- and intra-fraction organ changes. This in turn can lead to greater toxicity. The use of magnetic resonance imaging based adaptive radiotherapy (MRgART) endeavours to mitigate this variability to ensure adequate CTV coverage whilst minimising dose to adjacent Organs at Risk (OARs). The purpose of this study was to assess the utility of MRgART in the post-prostatectomy setting and to analyse toxicity in patients receiving treatment. Materials and Methods Nine patients were treated with MRgART utilising an Elekta Unity MR Linear Accelerator. Prescription used was 66-70Gy in 33-35 fractions with patients who had PSMA-detected local recurrences receiving a boost to the GTV. OAR doses were as per published guidelines. Daily adaptive plans were generated and clinically delivered. For each clinically delivered