ESTRO 2021 Abstract Book

S1147

ESTRO 2021

1 Medisch Spectrum Twente, Radiotherapy, Enschede, The Netherlands

Purpose or Objective In standard clinical practice image-guided radiotherapy (IGRT), based on rigid-body matching of the planning CT and the daily CBCT, is used to manage anatomical variations in prostate cancer patients. However, IGRT cannot completely compensate for patient-specific non-rigid anatomical variations such as organ deformation, rotation and motion between different organs. Online adaptive radiotherapy (oART) aims to deliver the radiation dose more accurately in the presence of such variations. Since July 2020, all prostate cancer patients in our department are treated with CBCT-guided oART. This study reports on the first fifty prostate cancer patients treated with CBCT-guided oART. Materials and Methods Between July 2020 and February 2021, fifty prostate cancer patients were treated with CBCT-guided oART (Ethos therapy, Varian Medical Systems, Palo Alto, CA). All patients were treated with a 9 or 12 field IMRT plan, with the aim to deliver 60 Gy in 20 fractions to the prostate and 54/60 Gy in 20 fractions to the seminal vesicles for more advanced stages. CTV-PTV margins of 7 mm in lateral direction and anterior-posterior direction and 8 mm in superior-inferior direction were used. The clinical workflow of oART consisted of CBCT acquisition, influencer (bladder, rectum, prostate and seminal vesicles) review, target review, comparison of scheduled and adapted plans, plan QA and delivery of the chosen plan. For all patients deviations in the planned workflow, treatment time, proportion of adapted fractions and fraction doses were analyzed. Results 16/20 fractions of one patient were excluded from analysis because the patient was treated using a backup IGRT plan after four fractions. From CBCT acquisition to end of treatment delivery, mean treatment time ± standard deviation was 14.5±1.8 minutes (range: 8.5-25.6 minutes). The adapted plan was selected in favor of the scheduled plan in 964/984 fractions (98%). Mean target coverage (V95% of PTV60Gy) of all fractions was increased for the adapted compared to the scheduled plans for 50/50 patients (100%). Mean bladder and rectum dose differences between adapted and scheduled plans exceeded 2% in 10/50 patients (20%) (Figure

1).

Conclusion We demonstrated that CBCT-guided oART is feasible within fifteen minutes and has the potential to improve target coverage, whereas cost to OARs was minimal for most patients. Analysis of accumulated doses of scheduled and adapted plans requires further investigation. PO-1397 Differences between various rectal volumes and implications for patient-reported rectal toxicity N. Pettersson 1,6 , C. Olsson 2,3 , T. Dang 2 , I. Olsson 4 , A. Valdman 5 1 Department of Medical Physics and Biomedical Engineering, Sahlgrenska University Hospital, Gothenburg, Sweden; 2 Department of Radiation Physics, Institute of Clinical Sciences, The Sahlgrenska Academy, Gothenburg University, Gothenburg, Sweden; 3 Regional Cancer Center West, The Western Sweden Healthcare region, Sahlgrenska University Hospital, Gothenburg, Sweden; 4 Umeå Institute of Design, Umeå University , Umeå, Sweden; 5 Department of Radiotherapy, G3:03, Karolinska University Hospital, Stockholm, Sweden; 6 Department of Radiation Physics, Institute of Clinical Sciences, the Sahlgrenska Academy Gothenburg University, Gothenburg, Sweden Purpose or Objective The purpose of this study was to investigate potential differences between rectal volumes defined according to the Swedish STRONG guidelines for male pelvis[1] 4 [2] 5 [3] 6 and rectal volumes defined in clinical practice for