ESTRO 2020 Abstract book

S961 ESTRO 2020

deformation due to bladder and rectum influencing the target structures. Planned versus delivered doses were computed for PTV, low risk clinical target volume (CTV LR ) and OARs such as bladder and rectum. For target structures, volume receiving 95% of the prescription dose V 95% (%), D mean (Gy) and D max (Gy) were evaluated. For OARs, volume receiving 40, 30 and 20Gy (V 40 , V 30 , V 20 ) and mean doses were evaluated. Dice similarity coefficient and mean Hausdroff mean distance also were evaluated to assess the quality of DIR. Dose delivered was 45 Gy in 25 fractions. Results 5/10 patients were qualified for evaluation, as the DIR was not of good quality and hence have to be excluded from the current study. Poor DIR quality may be attributed to large variation in rectum and bladder filling. Mean (±SD) volume of PTV, CTV LR of pCT was 1142(±295) cc and 242(±147) cc respectively. Mean (±SD) volume of bladder and rectum of pCT and CBCT were 230(±115) vs 293(±124) and 24(±6) vs 37(±12)cc respectively. Mean (±SD) DSC of bladder and rectum were 0.72(±0.06) and 0.64(±0.08), while mean (±SD) Hausdroff distance were 6.5(±1.2) and 4.4(±1.1)mm respectively. Mean(±SD) difference between the planned vs delivered dose to PTV, V 95% (%), D mean (Gy) and D max (Gy) were -3.3(±4.2), -0.03(±0.4) and - 0.3(±0.9) and that of CTV LR were 0.6(±1.3) , 0.4(±0.6) and -0.3(±0.5) respectively. Mean (±SD) difference between the planned vs delivered dose D mean (Gy), V 40 (%), V 30 (%) and V 20 (%) for bladder were -0.2(±0.8), -2.4(±2), -0.2(±3), - 0.3(±2) and that of the rectum were 0.1(±0.7), 0.6(±0.9), 0.9(±1.4) and 0.4(±0.9) respectively. Conclusion The difference between the planned and the delivered dose to target structures and OARs were quantified, which were found clinically acceptable. The quality of DIR is a major limitation for deformable dose accumulation in cervix cancer, especially when the deformation is large, influenced by bladder and rectum filling. PO-1658 Online-adaptive SRT with automated Organ At Risk contouring for abdominal oligometastases: Results J. Nuyttens 1 , L. Visani 2 , P. Granton 3 , M. Milder 4 1 Erasmus MC Cancer Institute, Radiation oncology, Rotterdam, The Netherlands ; 2 Azienda Ospedaliero- Universitaria Careggi- University of Florence, radiation oncology, FLorence, Italy ; 3 Erasmus MC Cancer Institute, radiation oncology, Roterdam, The Netherlands ; 4 Erasmus MC Cander Institute, Radiation Oncology, Rotterdam, The Netherlands Purpose or Objective In the phase II prospective STEAL trial, online-adaptive stereotactic radiotherapy (AO-SBRT) with a library of plans is used to treat patients with abdominopelvic lymphnode oligometastases. Here, we report the treatment workflow and plan choice of the first 12 patients. Material and Methods Eligible oligometastatic patients with abdominal lymphadenopathies were enrolled in the STEAL trial , and treated with CyberKnife R with an in-room CT scan (fig 1). Patients were all treated with a total dose of 45 Gy in 5 daily consecutive fractions of 9 Gy. For each patient, a library of 3 plans was created: plan A, based on the planning CT dose prescribed to the 90% isodose-line; plan B, with OARs contours based on their localization on a diagnostic CT scan; and plan C, based on the planning CT scan but with dose prescribed to the 80% isodose-line. Before each fraction, a CT-scan was made with the in-room CT scan and the patient remaining in the treatment position. Automatically, the fraction CT scan was forwarded to the image analysis software (MIM Software Inc. Cleveland) and a workflow initiated. The planning CT scan and the fraction CT scan were rigidly matched to one

The empty to half-full bladder plan was selected in 15% of the fractions in the first week and this proportion increased to 49% in the last treatment week (see Figure 2). The mean spared volumes of bowel bag for different dose levels (V45Gy, V42.74Gy, V40Gy and V30Gy) were 22.41cc, 21.51cc, 18.84cc and 11.86cc, respectively. The spared volumes were lower for the bladder: 0.81%, 3.48%, 3.11% and 1.87%. The mean CTV-T coverage (V42.75Gy) by the incorrectly selected plan was 98.72% for the half-full to full bladder plan and 99.8% for the empty to half-full plan.

Conclusion The mid position structures of the CTV-T can be used to generate a plan library. This strategy improves organ-at- risk sparing compared to a unique plan. As the method is time-consuming and the impact on toxicity is not yet known, the use of a plan library should still further be investigated. PO-1657 Deformable Dose Accumulation for patients treated with VMAT for cervical cancer. J. Swamidas 1 , R. Phurailatpam 1 , S. Chopra 1 , S. Panda 1 , K. Joshi 1 , U. Mahantshetty 2 1 Advanced Centre for Treatment Research and Education in Cancer ACTREC- Tata Memorial Centre, Radiation Oncology, Mumbai, India ; 2 Tata Memorial Hospital- Tata Memorial Centre, Radiation Oncology, Mumbai, India Purpose or Objective To evaluate the role of deformable image registration (DIR) and deformable dose accumulation (DDA) and to quantify the difference between planned versus delivered dose for patients treated with VMAT for Cervical cancer. Material and Methods Ten patients diagnosed with cervical cancer, with node negative status, who underwent EBRT using VMAT (Rapid Arc Eclipse TPS v13.5.37, True Beam v2.1, Varian Medical Systems) as a part of EMBRACE II (Image Guided Intensity Modulated E xternal Beam radio Chemotherapy and M RI based Adaptive Bra chytherapy in locally advanced Ce rvical Cancer), a multi centric international trial, were analyzed. Daily CBCTs were imported and deformedly registered to the planning CT (pCT) using CBCT corrected algorithm (Velocity v 3.2.1). Delivered dose was calculated by accumulating the dose using DIR and DDA. When accumulating the total dose, DIR accounted for