ESTRO 38 Abstract book

S989 ESTRO 38

The dose distribution in CTV was not essentially affected by the isocenter shifts and still fulfilled constraints to be used in our clinic, regardless of tumor location, dynamic technique used and shift direction. Only in few cases CTV constraints were slightly violated on average by: 0,7%, 1,1%, 0,9, 0,3% of re-calculated plans for D98%, D99%, D1% and D2%, respectively. In case of rectum (gynecology, IMRT), jaw (head and neck, VMAT) and lungs (lung, VMAT) overdosage caused by isocenter shift was observed in more than 7% of re-calculated plans. For bladder and pelvic bones (gynecology IMRT), jaw (head and neck IMRT), optic nerves, optic chiasm and brainstem (brain IMRT) discrepancies below 3.3 % of simulated plans were observed. More detailed information was included in Table 1. No relevant impact was observed on rest of OARs, important in analyzed tumor locations.

for clinical acceptability. A reduced number of MU was obtained for the robust plan with a mean variation of -18% (-32%/-10%) compared to the conventional plan. Very small variation of dose metrics for GTV and OARs were obtained considering the 3 dose accumulation methods, as well as the perturbed doses. Small differences non clinically relevant were found between 3D and 4D dose calculation for both robust and ITV-based plan.

Conclusion Based on 2400 simulated plans, dosimetric treatment plan parameters robustness was analyzed in the context of the daily patient positioning error. For all plans the dose distribution in the CTV fulfilled all investigated criteria. Analysis of OARs showed that in some cases isocenter displacement cause dose restrictions exceeding, however it is the worst-case scenario, when the systematic error would occur. In case of serial OARs, for which PRV concept was used, the maximum overdosage not exceed more than 1 Gy. Thresholds used in our clinic in patient position verification seem to be well adjust as to treat patients properly. EP-1824 Hybrid-Volumetric Modulated Arc Therapy in the Upper Thoracic Esophageal Cancer: A Planning Study Y.E. Choi 1 , K. Sung 1 , H.J. Kim 1 , Y.K. Lee 1 1 Gil Medical Hospital, Radiation Oncology, Incheon, Korea Republic of Purpose or Objective To compare the dosimetric differences and lung sparing of three different treatment techniques – three-dimensional conformal radiotherapy (CRT), volumetric modulated arc therapy (VMAT) and Hybrid-VMAT (H-VMAT) in the treatment of upper thoracic esophageal cancer. Material and Methods CRT, VMAT, and H-VMAT plans were regenerated for 14 upper thoracic esophageal cancer patients with T3N0-1M0. The target prescription dose was 50.4 Gy. All plans were optimized to reach clinically acceptable levels by the departmental plan criteria: the maximum dose for the spinal cord was less than 45 Gy, whereas the volume of the lung irradiated by a dose of 20 Gy (V 20 ) and 30 Gy (V 30 ) was less than 30% and 20%, respectively. For PTV coverage, 95% of PTV should be covered by 95% of the prescription dose, unless the spinal cord limit was violated. Plan quality was evaluated using: conformity index (CI), homogeneity index (HI), mean lung dose (MLD), lung volume receiving >5-30 Gy (lung V x ), and maximum spinal cord dose (cord D max ). The Wilcoxon Signed-Rank test was used to determine any differences between datasets with a Bonferroni’s adjustment for multiple comparisons ( p < 0.017). Electronic Poster: Physics track: Treatment planning: applications

Conclusion Robust optimization to account for respiratory motion was investigated for DWA lung SBRT. Our preliminary results confirmed the robustness against realistic variation in respiratory motion. Further investigations are warranted to confirm the clinical effectiveness of this novel approach compared to the conventional margin-based approach. EP-1823 Analysis of treatment plans robustness for dynamic techniques in external beam radiotherapy E. Dabrowska-Szewczyk 1,2 , M. Bukat 1 , P. Kukolowicz 1 , D. Szalkowski 1 , A. Paciorkiewicz 1 , A. Zawadzka 1 1 Maria Skłodowska-Curie Memorial Cancer Center and Institute of Oncology, Department of Medical Physics, Warsaw, Poland ; 2 University of Warsaw, Department of Biomedical Physics, Warsaw, Poland Purpose or Objective Uncertainty of patient positioning during treatment session have a significant impact on dose distribution in clinical target volume (CTV) and organs at risk (OARs). However, usually some discrepancy between DRR and daily set-up of the patient’s position during the treatment are acceptable. The aim of our study was to evaluate the robustness of dynamic techniques dose distributions against isocentre shifts and to verify whether threshold used in patient position verification were chosen correctly. Material and Methods For this study, plans prepared for 400 patients treated with VMAT or IMRT were retrospectively analyzed (50 plans per the following cases: brain-IMRT, head and neck- IMRT, head and neck-VMAT, breast-IMRT, breast VMAT, lung-VMAT, gynecology-IMRT, canal anal-VMAT). To estimate the robustness of analyzed plans against simulated isocenter shifts, tool named “Plan Uncertainty” was used (Eclipse TPS, Varian, version 15.6). For each plan the new 6 dose distribution were re-calculated with taking into account the positive and negative maximally acceptable threshold in our clinic during patient position verification. The values of acceptable shifts are as follow: 3 mm in case of brain and head and neck patients, 6 mm in case of breast patients and 4 mm in case of lung, gynecology and anal canal patients. For 400 references and 2400 re-calculated plans, near to minimum doses (D98%, D99%) and near to maximum doses (D1%, D2%) for CTV were investigated. Tolerance doses for relevant OARs were also evaluated according to our clinical protocol. Results