ESTRO 37 Abstract book

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ESTRO 37

Results A total of 128 cine-MR series originating from 11 patients were analyzed. The largest tumor motion was found in CC direction, with a mean peak-to-peak motion of 10.9 mm (standard deviation [SD] 4.9 mm), followed by AP with 3.5 mm (SD 1.9 mm) and LR with 2.6 mm (SD 1.4 mm). Large variations in intra-fraction motion were found within and between patients. The presence of a severe inhale was found in 5 of the 128 scans (4%). After exclusion of these scans mean peak-to-peak motion was 10.3 mm (SD 3.8 mm). Correction for drift showed a reduction in mean peak-to-peak motion to 9.5 mm (SD 3.3 mm). Reduction of the motion is visualized in Figure 2. Despite correction, large variations in tumor motion between measurement points within patients was observed.

linac. This system uses external markers to control the respiratory cycle and internal fiducial markers to measure the movement of the tumor and set up the patient. The tumor position is measured intermittently during the treatment via stereoscopic x-ray images to compensate the baseline drift. Therefore, the accumulative changes in the couch position correspond to the baseline drift in the tumor motion. Results The average change in position of the treatment couch during the treatment time was 0.2 ± 0.8 mm (mean ± standard deviation), -0.7 ± 2.1 mm, and 2.3 ± 5.3 mm in the left-right (LR), antero-posterior (AP) and cranio- caudal (CC), directions respectively. Overall the baseline shift/drift occurs toward the posterior and cranial directions. The incidence of a baseline drift exceeding 3 mm was 0% and 22%, for the AP and CC directions respectively, within 10 minutes of the start of treatment, and 43% and 86% within 25 minutes. On the other hand, the incidence of a baseline drift exceeding 5 mm was 0% and 10% for the AP and CC directions respectively, within 10 minutes of the start of treatment, and 14% and 71% within 25 minutes. The intra-fractional uncertainties due to baseline drift of liver tumors are: Direction M (mm) Σ (mm) σ (mm) LR 0.1 0.2 0.4 AP -0.3 0.6 0.8 CC 1.2 2.2 1.5 M: the overall mean or group systematic error, Σ: the standard deviation (SD) of the systematic error, σ: the SD of the random error. In the absence of intra-fractional IGRT, the baseline drift uncertainties imply the use of increased margins of up to 4 mm and 3 mm in the cranial and caudal directions respectively, in SBRT for liver tumors, when the rest of uncertainties are minimized. Conclusion Real-time monitoring and frequent adjustments of the couch position are suggested to be essential to compensate for possible underdosage in CC direction due to baseline drift in SBRT for liver tumors. EP-2030 Examination of the automatic fiducial marker detection on the Vero system M. Ziegler 1 , T. Brandt 1 , S. Lettmaier 1 , R. Fietkau 1 , C. Bert 1 1 University Hospital Erlangen, Radiation Oncology, Erlangen, Germany Purpose or Objective The Vero SBRT system (Brainlab, Munich, Germany) can treat intra-fractionally moving tumors via dynamic tumor tracking (DTT). During a DTT treatment, the tumor position is determined by extracting the 3D-position of implanted radio-opaque fiducial markers (FM) from two orthogonal kV X-ray images. At the University Hospital Erlangen two Visicoil markers (IBA, Schwarzenbruck, Germany) with a diameter of 0.75 mm and a length of 20 mm are implanted in close vicinity to the tumor one week before pre-treatment imaging and used as FM during DTT. In this work the detection rate of the FM at the Vero system is investigated and possible influencing Nine male and two female patients between the age of 51 and 90 years were treated with DTT at the University Hospital in Erlangen with a total of 132 fractions. On median 173 X-ray image pairs were taken during every fraction of the treatment. In 64.6 % of these, the FMs are detected by the Vero system. For each kV image the average intensity I of the tissue surrounding the FM in a 3 factors are determined. Material and Methods

Conclusion Intra-fraction motion was found to be highly variable between patients. Furthermore, motion within patients was found to be variable over the different measurement points. Correction for severe inhales and drift reduced motion amplitudes. This indicates that real-time tumor motion management during radiotherapy is desired to safely reduce margins. EP-2029 Intrafractional baseline drift in SBRT of liver 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 , X. Chen Zhao 2 , B. Alvarez Rodríguez 2 , R. Ciérvide Jurio 2 , J. Valero Albarrán 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 liver tumors in stereotactic body radiation therapy (SBRT), using a gating respiratory system radiation therapy with intra-fractional IGRT. Material and Methods 55 fractions of 16 patients with liver tumors were treated with SBRT using ExacTrac Adaptive Gating® in a Novalis®