ESTRO 2021 Abstract Book

S1668

ESTRO 2021

Materials and Methods The patient selection was four patients with left-sided BC and PE, who were considered for radiation therapy at Oslo University Hospital (OUH) during 2014-2016. Comparative treatment plans were created with the purpose of delivering 40.05 Gray (Gy) over 15 fractions with photons in 3D-CRT and hybrid-VMAT techniques, and protons with IMPT. Robust optimization was performed in the IMPT plans. The treatment plans were assessed by the dose to the clinical target volume (CTV) as well as the volume- and mean doses to the heart and ipsilateral lung based on the treatment planning guidelines at OUH as following; Dose to 98% of the volume (D98) for CTV >95%, mean heart dose (MHD) <2 Gy, volume of left lung receiving 18 Gy (V18Gy). Conformity Index (CI) representing volume of 95% isodose divided by CTV volume and the dose to right lung were also assessed. Results All treatment plans had satisfactory dose to the CTV. The 3D-CRT plans had the lowest CTV dose with D98-values between 95% and 95,8%. In IMPT plans, D98-values were between 95,8% and 97,2%. The hybrid-VMAT plans had the highest CTV dose with D98-values between 96,4% and 98,1%. The best CI of the CTV was found in the IMPT-plans with an average CI of 1,4 compared to 3,1 and 3,0 for 3D-CRT and hybrid-VMAT, respectively. The 3D-CRT and hybrid-VMAT plans had relatively similar doses to the heart, with great differences between the patients. The MHD in the 3D-CRT plans had between 5,8 Gy and 18,2 Gy, while the hybrid-VMAT plans had MHD between 6,3 and 19,4 Gy. The lowest MHD were found in the IMPT plans showing doses between 0,9 and 1,4 Gy. For the left lung, the IMPT plans had the lowest doses of V18Gy between 10,2% and 21,1%. The 3D-CRT plans showed V18Gy between 29,6 and 47,3%, while the hybrid-VMAT plans resulted in V18Gy between 29,8 and 46,9%. Conclusion The IMPT-plans achieved adequate dose to the CTV, as well as reducing the heart and lung dose, compared to the 3D-CRT- and hybrid-VMAT-plans. Proton therapy can potentially give a better dose distribution in patients with PE who are referred for RT for left sided BC. Due to the limited number of patients in this study, more research is required in order to conclude any further. PO-1960 Setup error-related dosimetric uncertainties on GTV in SBRT Y. Jeong 1 , J.G. Oh 1 , K.K. Lee 1 , S.R. Moon 1 1 Wonkwang University Hospital, Wonkwang University School of Medicine, Department of Radiation Oncology, Iksan, Korea Republic of Purpose or Objective To evaluate the setup error-related dosimetric uncertainties on gross tumor volume (GTV) in stereotactic body radiation therapy (SBRT). Materials and Methods For 10 SBRT plans for lung cancer (n=5) and hepatocellular carcinoma (n=5), 26 scenarios of plan uncertainties, which were generated by the combination of setup errors of 0 mm, +5 mm, or -5 mm in the X, Y, and/or Z directions, were calculated by plan uncertainty tools of Eclipse treatment planning system. In the nominal (original) plans for all patients, planning target volume (PTV) was defined as the expansion of internal target volume with 5 mm margins, and 95% of the PTV were received ≥ 95% of the prescribed dose. Dose-volume histogram (DVH) parameters, D98%, D50%, and D2% for GTV, in 26 scenarios were compared with those in the nominal plans. Results In the nominal plans, D98%, D50%, and D2% for GTV were median 101.1 % (range, 96.2-103.1%), 102.8% (range, 99.9- 104.5%), and 104.8% (range, 102.9-106.3%) of prescribed doses, respectively. In comparisons of 26 scenarios of plan uncertainties with the nominal plans, percentage differences in D98%, D50%, and D2% were median -1.7% (range, -42.7- 5.5%), 0% (range, -5.7-5.0%), and 1.3% (range, -3.4-10.3%), respectively. After excluding 1 plan for lung cancer which was abutting to the chest wall, percentage differences in D98%, D50%, and D2% were median -1.7% (range, -16.8-4.0%), 0% (range, -5.7-5.0%), and 1.3% (range, -3.4-10.3%), respectively. Conclusion The setup error seems to be related to the perturbation of dose coverage for GTV in SBRT plan, even though the ranges of setup errors were within the margin of the PTV. Further studies are needed to evaluate the setup error-related dosimetric robustness in SBRT plan. PO-1961 The effect of 4DCT vs. 3DCT simulation on the positioning accuracy for lung cancer radiotherapy Y. Wang 1,1 , P. Yang 1 , Y. Liu 1 1 Peking University Shenzhen Hospital, Radiation Oncology, Shenzhen, China Purpose or Objective To investigate the difference of setup errors between 4DCT and 3DCT simulation in the treatment positioning accuracy of patients undergoing lung cancer radiotherapy. Materials and Methods 12 patients with lung cancer were retrospectively selected in this study. Among them, half patients went through 4DCT simulation, and the other half went through regular 3DCT simulation. All CT simulations were performed on Siemens large- bore CT scanner with slice thickness of 3 mm and resolution of 512x512. Subsequently, both groups had cone beam CT (CBCT) as image guided radiotherapy (IGRT) prior to radiation treatment. There were 88 CBCTs acquired and registered with planning CT in the 6-patient 4DCT simulated group. There were 53 CBCTs acquired and registered with planning CT in the 6-patient 3DCT simulated group. All the IGRT positioning accuracy and set-up error was verified by radiation oncologists, medical physicists and radiation therapists. All patients setup was head first, supine on vaclok with both arms placed up around the top of skull. Results The setup errors in lateral, vertical, longitudinal and couch rotational directions using 4DCT and 3DCT simulation were as below in Table1. There was almost no difference in the lateral shift. However, the vertical shift, longitudinal shift and Digital Poster: Image-guided radiotherapy and verification protocols