Abstract Book

S1111

ESTRO 37

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 approach can be applied to stereotactic radiosurgery and other high precision radiotherapy techniques. Material and Methods The system is comprised of an array of conductive sensors arranged around the patient’s cranium during the SRS treatment. Finite Element Analysis (FEA) was employed to determine a viable sensor array arrangement for 3D detection. Capacitance of the sensor array is monitored continuously and calibrated to patient position. The system is unique in that it is not sensitive to the position of the thermoplastic mask but registers the motion of the cranium.

Using an experimental prototype, the first phase of development was focused on detecting translational motion of the cranium. Prototype testing was conducted with the help of a volunteer in a thermoplastic mask. Experiments were performed using a hexapod stage capable of performing 3D motion to simulate patient motion by moving the sensor array relative to the cranium. Four copper sensors (4 by 6 inch) were used in the arrangement shown in figure 1 around the volunteer’s cranium. Motion was introduced in millimeter steps and data were gathered for 3 seconds at each position for all sensors. The average sensor response and the standard deviation were calculated and plotted. Results The results show a sensitivity of over 4 counts/mm for the first 4 mm of motion in the superior direction and a differential signal change of over 10 counts/mm for lateral direction derived from the two lateral sensors. The results of lateral and superior-inferior motion are shown in figures 1 and 2, respectively, demonstrating detection of both superior-inferior and lateral motion with sub millimeter accuracy. Notably, motion in a given direction only affects the signal along the relevant axis, without significant cross-talk. Conclusion The technology described here shows promise in providing real-time, continuous, and sub-millimeter 3D monitoring of the cranium without the need for ionizing radiation or the use of skin as a surrogate.

EP-2035 Intrafractional baseline drift in SBRT of pancreas tumors J. García Ruiz-Zorrilla 1 , M.A. De la Casa de Julián 1 , C. Rubio Rodriguez 2 , P. Fernández Letón 1 , J. Martí Asenjo 1 , D. Zucca Aparicio 1 , O. Hernando Requejo 2 , M. López González 2 1 Hospital Universitario HM Sanchinarro, Radiofísica, Madrid, Spain 2 Hospital Universitario HM Sanchinarro, Oncología Radioterápica, Madrid, Spain Purpose or Objective To investigate the frequency and amplitude of baseline or drift of pancreas tumors in stereotactic body radiation therapy (SBRT), using a gating respiratory system radiation therapy with intra-fractional IGRT. Material and Methods 76 fractions of 15 patients with pancreas tumors were treated with SBRT using ExacTrac Adaptive Gating® in a Novalis® linac. This system uses external markers to