ESTRO 2021 Abstract Book

S1268

ESTRO 2021

M. Bak 1 , J. Friborg 1 , N. Jensen 1 , F. Kjær-Kristoffersen 1 , T. Nøttrup 1 , H. Roed 1 , H. Mathiesen 1 , M. Sjölin 1 , I. Vogelius 1 1 Rigshospitalet, Oncology, Copenhagen, Denmark Purpose or Objective Vulvar cancer is a rare gynecologic malignancy. Postoperative radiotherapy for vulvar carcinoma is challenging due to a high burden of radiation sequelae, relatively high risk of locoregional disease recurrence and technically challenging target combined with common postoperative lymphocele causing anatomical variations. These factors have made postoperative vulvar carcinoma a preferred patient group for our very early experiences with online adaptive radiotherapy on a dedicated Ethos machine. We report on the dosimetric gain between the scheduled and adaptive plan and present data on the preservation of such dosimetric gains at the verification scan immediately prior to treatment. Materials and Methods First adaptive radiotherapy fraction at our institution was given Jan. 22 2021. At present, two postoperative vulvar cancer patients have been treated and the adaptive plan was chosen in 13 of 18 possible fractions. The workflow and dose calculation process is depicted in Figure 1. We report the following data on five adapted fractions • Time from setup CBCT scan to verification CBCT (Adaptation time) • Dose volume data from scheduled plan on the setup CBCT • Dose volume data from adapted plan on the setup CBCT • Dose-volume data from adapted plan on verification CBCT

Results Average adaptation time is 16 minutes (Range 12-20 minutes), cf. Figure 1. DVH parameters for Rectum and bladder dose to the hottest 2% along with PTV and CTV dose to 95% of structure are depicted in Figure 2. Improvements in CTV coverage appear preserved during adaptation whereas normal tissue data is mixed.