ESTRO 38 Abstract book

S1069 ESTRO 38

Material and Methods 19 patients with a single peripheral lung tumor, smaller than 30 cc, were included in this study. Two different immobilization systems were used: 9 patients were immobilized using the Elekta WingSTEP device (system1) and 10 patients using the Qfix Arm Shuttle with Vac-Qfix vacuum bag (system2). For each patient a 4dimensional- CT was acquired in a Philips Brilliance Big Bore CT. The ITV was delineated according to six breathing phases. In total, 35 fractions were delivered in system1 and 32 fractions in system2. Two risk risk-adapted fractionation schemes were used: 3 fractions of 18 Gy and 5 fractions of 10 Gy. For each fraction, three 4-dimensional Cone Beam CT (4D-CBCT) scans were acquired: before treatment to measure and correct the mean tumor position (initial set- up), after correction to validate the correction applied, and after treatment to estimate the intrafraction stability. (In system2, after correction 4D-CBCT was no acquired). These scans were volume registered with the localization CT, using a soft-tissue match, in the Elekta XVI software. Corrections were performed by a robotic patient positioning platform that enables sub-millimeter accuracy (Elekta HexaPOD evo RT system). Treatment was delivered using 6 MV photons generated from an Elekta Synergy Beam Modulator Linac. Patient positioning data from all scans were recorded to determine systematic ( Ʃ ) and random (σ) set- up errors for initial set-up, after correction and after treatment imaging, in the left-right (X), craniocaudal (Y) and anteroposterior (Z) directions. The ITV to PTV margin (M) was also calculated for after correction and after treatment imaging, using the Van Herk formula: M=2.5 Ʃ +0.7 σ Results A summary of patient set-up errors, intrafraction stability and ITV to PTV margins, in the three orthogonal directions, is shown in Table I. Calculated margins do not account for target delineation and organ motion uncertainties and are consistent with our current 7 mm margins in all directions. Conclusion Patients treated using frameless SBRT are well immobilized in both systems studied. This is demonstrated with small intrafraction movements after initial set-up correction using imaging guidance. Therefore this technique can be safely administrated using 4D-CBCT. ITV margins are smaller and intrafraction stability improves using the Vac-Qfix immobilization system compared to WingSTEP device. Vac-Qfix immobilization system can be more convenient for patients with certain clinical conditions. EP-1961 Evaluation of dosimetric and anatomic parameters in Deep Inspiratory Breath Hold breast Radiotherapy A. Nachankar 1 , A. Pawar 1 , A. Jadhav 1 , N. Burela 1 , A. Kakade 1 , P. Dandekar 1 1 Sir H N Reliance Foundation Hospital and Research Center, Radiation Oncology, Mumbai, India Purpose or Objective Cardiovascular morbidity following radiotherapy to left sided breast cancers is significant. Several options have

Purpose or Objective Dynamic tumor tracking (DTT) is an advanced treatment technique to treat intra-fractionally moving tumors. Until now, the CyberKnife (Accuray Inc., Sunnyvale, CA, USA) and the Vero 4DRT system (BrainLab AG, Munich, Germany) are the only two commercially available systems which can perform a DTT treatment. Both systems detect the internal target position from X-ray images. Until recently, the Vero system required implanted fiducial markers to extract the target position from the X-ray images. Recently, marker-less DTT (MLDTT) was introduced to the Vero 4DRT system with ExacTrac 3.6.1. This work evaluates the accuracy of the new MLDTT method of the Vero 4DRT system and reports the results of the first patients treated with MLDTT at the Vero system. Material and Methods The BrainLab Gating Phantom was used in combination with a QA-tool provided by the vendor (an inhomogeneous "egg" made from a gypsum-like material on an acrylic plate) to measure the detection accuracy of the new MLDTT software. The phantom consists of a vertical and a horizontal one-dimensional drive. A patient breathing curve was used as an input for the phantom and the accuracy was measured for all three spatial directions of the Vero system. Three patients have been treated with MLDTT at the Vero system of the University Hospital Erlangen. During a MLDTT treatment additional kV images are acquired to determine a template of the day used for tumor detection during treatment delivery. The additional dose delivered by the additional images was determined using the CTDI Set for CT Dosimetry (PTW, Freiburg, Germany) and a log- file analysis was performed to evaluate the MLDTT treatment. Results The accuracy of the MLDTT detection is 0.12 mm ± 0.12 mm, 0.12 mm ± 0.11 mm and 0.20 mm ± 0.21 mm for the x-, y- and z-direction, respectively which is comparable to the accuracy of the marker-based DTT target detection (0.08 mm ± 0.09 mm, 0.07 mm ± 0.08 mm and 0.07 mm ± 0.06 mm). The median treatment time for the MLDTT patients was 18 min 25 seconds. On median 65 additional kV images were acquired per treatment fraction to determine the template for the target detection. In comparison to DTT, this results in an additional isocenter dose of 257.4 mGy over the course of the entire treatment. Conclusion Three patients were treated with MLDTT at the University Hospital Erlangen. The accuracy of the MLDTT target detection and the treatment time of a MLDTT treatment were comparable to the marker based marker DTT of the Vero system. The additional imaging dose administered during the treatment can easily be justified since it spares the imaging dose of a CT guided marker implantation into the lung which is an interventional procedure carrying a great risk of pneumothorax. EP-1960 A comparison of positioning accuracy for frameless lung SBRT using two immobilization systems J.M. Penedo Cobos 1 , J. Luna 1 , M.A. Garcia 1 , E. Lopez 1 , K. Aguilar 1 , R. Gonzalez 1 , A. Sanchez 1 , S. Gomez-Tejedor 1 , M. Rincon 1 , M. Alarcia 1 , S. Martin 1 , D. Gonsalves 1 , J. Olivera 1 1 Capio-Fundación Jimenez Díaz, radiotherapy oncology, Madrid, Spain Purpose or Objective To quantify the localization accuracy and intrafraction stability of lung cancer patients treated with frameless stereotactic body radiotherapy (SBRT) using two different immobilization systems and to calculate the Internal Target Volume (ITV) margins in both cases to account for the setup errors.