ESTRO 2020 Abstract Book

S889 ESTRO 2020

delivered dose using radiotherapy planning (RTP) CT and cone beam CT (CBCT) acquired in 10 patients during cervix EBRT. Target (CTV) and OARs, were manually segmented on 90 datasets and ART strategies modelled were: 1. ITV+ standard margins based on RTPCT and diagnostic MRI (clinically delivered plan) 2. CTV+ standard margins based on radiotherapy planning CT (RTPCT-only) 3. Partial uterine irradiation: CTV defined from GTV on RTP MRI+15 mm (Small CTV) 4. Plan of the day, using CTV defined on full, mid and empty bladder volume RTP imaging (POD) Composite target volume defined from CBCTs acquired days 1-3 Daily online re-planning using CTV defined on CBCT acquired at treatment +3 mm PTV margin Daily online re-planning using CTV defined on CBCT acquired at treatment +5 mm PTV margin CBCT electron density was established by bulk density assignment and dose for each plan was calculated on the weekly CBCT. Computed deformable image registration was used to deform the CBCT and its dose to the RTPCT. All plans were quality assured for standard target and OAR planning objectives. The simulated delivered dose to the target and OARs was summated and compared for different treatment strategies using Friedman’s test with Dunn’s test for multiple comparisons. Results Underdosing of CTV occurred in one patient for the RTPCT only, D1-3 and partial uterine irradiation non-adaptive plans, where significant inferior CTV motion occurred due to bladder and rectal volume change. Compared to the clinically delivered plan, ART resulted in OAR sparing, particularly at higher doses, with online re- planning providing maximal OAR sparing. Table 1 and figure 1 illustrate increasing OAR sparing observed with increasing sophistication of adaptation. Largest reductions were seen with online 3 mm PTV margin strategy with standard and small CTV. Compared to the clinically delivered plan, POD resulted in 44% and 70% reduction in rectum and bladder V4500 cGy. There were statistically significant differences in delivered V4000 cGy to rectum and bladder for all online strategies with re-planning, when compared to the clinically delivered plan. 5. 6. 7. 8. Daily online re-planning defined on CBCT acquired at treatment using partial uterine irradiation MRI-CTV +3 mm PTV margin

Conclusion ART solutions for cervix EBRT reduce OAR doses. Daily online re-planning delivers the largest OAR dose reductions. Clinical studies combining MRI-guided ART and daily online re-planning are now required to assess clinical benefit. PO-1627 Evaluation of Tumor Volume and Electron Density Changes on CBCT Images in Lung Cancer Patients E. Kara 1 , O. Ozdemir 2 , A. Hicsonmez 1 , F. Zorlu 3 1 Onko Ankara oncology center, Oncology Department, Ankara, Turkey ; 2 Koru Ankara Hospital, Radiation Oncology, Ankara, Turkey ; 3 Hacettepe University, Radiation Oncology, Ankara, Turkey Purpose or Objective The aim of this study is to evaluate the volumetric changes on gross tumor volume (GTV) during the treatment and to determine the changes in the irradiated tumor volume and the electron density values of the normal ipsilateral lung and GTV by using kilo-voltage cone beam (kV-CBCT) in lung cancer patients. Material and Methods In this study, 10 patients with lung cancer, whose tumor location was different from each other and the volume changes could be observed by CBCT during the treatment were selected for this study. CBCT images obtained in fractions 1, 15 and 30 were examined to detect changes in volume and electron density for GTV. The electron density profiles of the tumor volume and ipsilateral lung in the 1st, 15th and 30th fractions were obtained. CBCT images were transferred to the treatment planning system and the plans were re-examined on these images to determine the effect of inter-fraction electron density change on dose distribution. Results As a result of our study, GTV volume and maximum electron density decreased throughout the fractions.