ESTRO 37 Abstract book

S1113

ESTRO 37

between the gating and the delivery system) were performed. The lesion selected was a thymoma: Patient1 treated with 66 GyRBE, Patient2 with 54 GyRBE. An amplitude based reconstructed 4DCT (10 phases) and a free-breathing CT (FBCT) were acquired and used for the planning. The physician copied the CTV from each phase of the 4DCT to the FBCT to create the ITV. By using the deformable registration, each OAR was mapped and verified from the FBCT to each phase of the 4DCT. The planning was performed on the FBCT. The approved plan was evaluated in two ways: - It was recalculated on each phase to find the worst case scenario in terms of OAR overdosage - Interplay effect: every spot was assigned to a phase accordingly to the beam delivery and the breathing curve of the patient. The 10 doses were then deformed on the FBCT and summed to evaluate if the dose homogeneity to the target and the constraints to the OAR were deteriorated and violated. During treatment delivery accurate gating was ensured allowing automatic beam hold when the breathing amplitude was higher than the one measured during 4DCT scan. Results The validation of the gating technique ensured a good quality of the beam delivery with out-of-threshold interruptions. The timing between the gating off signal and the interruption of the beam was always below 90ms (maximum allowed was 100ms). The evaluation of dose on each phase and of the interplay effect gave the results shown in table for both patients. The coverage of the ITV was always ensured and the OARs were always within the constraints: no interplay effect was highlighted. The second patient showed an overdosage to the coronary and Left Anterior Descending area respect to the nominal plan but it was always below the constraints. The table shows the average and the worst Duty Cycle (DC) per fraction during the entire treatment. Two examples of breathing curves are provided in figure. The breathing of the first patient is stable and always within the gating thresholds, whilst for the second patient more amplitude and phase variations are present, with multiple beam interruptions due to the out-of- threshold amplitude signal.

Conclusion We defined, organized, validated and used for two patients a protocol for the treatment of small amplitude moving targets. The planning and delivery of the treatments gave very good results in terms of coverage, OARs sparing, 4D dose evaluation of the plan and interplay effect assessment. EP-2033 Determining intra-fraction variation in image guided lung SBRT based on 4DCBCT M. Simon 1 , I. Balogh 1 , E. Dobos 1 , A. Kovács 1 , G. Hócza 1 , E. Csiki 1 , E. Szántó 1 , A. Kovács 1 , P. Arkosy 1 1 University of Debrecen Med. and Health Science Centre, Department of Radiotherapy, Debrecen, Hungary Purpose or Objective The role of stereotactic body radiation therapy in the treatment of early stage non-small cell lung cancer is increasing. Including intra-fraction variation (IFV) into the treatment margin definition has high importance. The aim of this study was to analyze IFV in our SBRT patient cohort. Material and Methods Between November 2015 and July 2017 a total of 40 patients received lung SBRT treatment in 324 fractions based on 4DCT. Translational position correction was carried out with on-board CBCT using 4DCBCT preset. Post fraction 4DCBCT scan was used to determine IFV data for each fraction. 648 CBCT scans were recorded to determine geometrical uncertainties and treatment planning margins for lung SBRT. All registration was performed on XVI 5.0 (Elekta, Stockholm, Sweden). Results Overall population mean set-up error (±population random error) for pre-treatment corrections were: - 0.01(±0.35) cm, +0.05(±0.34) cm and -0.13(±0.30) cm in Superior-Inferior (SI), Left-Right (LR) and Anterior- Posterior (AP) directions respectively. Post fraction mean set-up errors (±population random error) were - 0.02(±0.08) cm, +0.07(±0.1) cm and -0.04(±0.07) cm in SI, LR and AP directions respectively. The population systematic error components were +0.07 cm, +0.12 cm and 0.1 cm in SI, LR and AP directions respectively. Conclusion 4DCBCT is appropriate method to define IFV and estimated margins can be obtained. Combining these components with other systematic components can help to develop adequate target margins for lung SBRT. EP-2034 Capacitive monitoring system for intra- fraction motion detection during frameless radiosurgery P. Sadeghi 1 , J. Lincoln 1 , E. Ruiz 1 , J. Robar 1 1 Dalhousie University, Physics & Atmospheric Science, Halifax, Canada Purpose or Objective This work presents an innovative technology for continuous intra-fraction 3D patient position monitoring with sub-millimeter accuracy. The system provides real- time motion monitoring without the use of ionizing radiation or relying on surrogates such as skin. The