ESTRO 38 Abstract book

S1071 ESTRO 38

Pérez González 1 , M.T. Murillo González 3 1 University Hospital La Princesa, Medical Physics, Madrid, Spain ; 2 University Hospital La Princesa, Radiation Therapist, Madrid, Spain ; 3 University Hospital La Princesa, Radiation Oncology, Madrid, Spain Purpose or Objective Patient motion and breathing cause misalignment of the tumor and toxicities to the healthy tissue during lung Stereotactic Body Radiation Therapy (SBRT). In order to ensure the accuracy of dose delivering, correction motion can be applied by using Image-Guided Radiation Therapy (IGRT). The aim of this work is to describe our IGRT Data from 31 patients undergoing SBRT for lung cancer have been analysed for this study. Our IGRT protocol involves the use of time-resolved four-dimensional CT (4DCT) scanning technique (Real-Time Position Manager by Varian) in a Toshiba AquilionLB unit in the simulation phase, and the use of Cone Beam Computed Tomography (CBCT) and fluoroscopy images (On-Board Image by Varian) in the treatment phase in a Clinac 2300 iX. - Simulation phase: during the scan, the patient is instructed to breathe normally. A 4DCT scan of 10 respiratory phases is reconstructed using phase sorting. After that, contour of the target volume in the 10 sets of CTs is performed. The used approach is creating an internal target volume (ITV) which encompasses the entire tumor displacement in a breathing cycle. ITV is expanded with a margin of 5 mm to create the PTV. - Treatment phase: radiation delivery consists on three coplanar dynamic hemi-arc fields with RapidArc. Before dose deliver, a full rotation pre-CBCT is performed in the OBI system. Our deviation tolerance in the tumor from the reference setup, determined in three dimensions, is 1cm. Between field activation, intra-fraction anterior-posterior projection fluoroscopy imaging is performed to confirm tumor motion within the PTV contour. If the tumor is not visualized in the fluoroscopy image or the motion exceeds the PTV contour, intermediate CBCT image is performed. Finally, after treatment a post-CBCT assesses intra- fraction tumor displacement. Results For the group of 31 patients, the 84% were able to be performed intra-fraction fluoroscopy against 16% in which the image contrast was low for visualization of the tumor. By using fluoroscopy imaging tumor motion within the session was verified in 61% of the patients. On the other hand, fluoroscopy images showed intra-fraction tumor displacement, and therefore the need of reposition, in 23% of the patients (Figure 1). The average and standard deviation of the setup errors for all the patients are showed in Table 1. Pre-CBCT presents larger setup errors than post-CBCT, since tumor position is verified, and corrected if necessary, with the fluoroscopy imaging. Protocol for Lung SBRT. Material and Methods

Conclusion IGRT is an important motion management strategy for tumor motion, as in SBRT lung cancer. Fluoroscopy manages to verify tumor position accuracy and in comparison to CBCT intra-fraction, it involves reducing treatment duration and minimizing dose to patient. Even though fluoroscopy depends on high contrast for visualization, in conjunction with CBCT, it may be used to confirm motion during the treatment. EP-1964 Setup verification and Intrafraction motion monitoring with Optical Surface Imaging for frame-less SRS A. Nachankar 1 , A. Pawar 1 , A. Jadhav 1 , P. Dandekar 1 1 Sir H N Reliance Foundation Hospital and Research Center, Radiation Oncology, Mumbai, India Purpose or Objective Accurate positioning, rigid immobilization and image verification is crucial in high precision stereotactic radiosurgery. We evaluated setup accuracy using frameless open mask stereotaxy system with optical surface monitoring (OSMS) as an adjunct to gold standard Cone Beam CT (CBCT) scan. Material and Methods We retrospectively analyzed 12 patients treated with frameless cranial stereotactic radio surgery (SRS) during August 2016 and June 2018, at our institution. Patients were immobilized using Q fix immobilization system (Q-fix Aquaplast mask, Head & Neck base plate, MOLDCARE® Cushion & Silverman head support). All patients were aligned to the isocenter with lasers on Perfect Pitch 6 DoF couch. Set up errors were calculated by OSMS prior to acquiring CBCT by comparing surface data to planning CT scan in translational and rotational directions. All the patients underwent CBCT based for correction of setup errors pretreatment and posttreatment verification CBCT. OSMS continuously monitored for intrafraction motion and threshold for beam holding for translational motion was 3 mm and rotational motion was 2 degrees. Data was analysed by Paired T test and Pearson correlation coefficient. Results A total of 24 CBCT scans were analyzed for 12 patients undergoing cranial SRS. Setup errors documented by CBCT were compared with the shifts computed by OSMS. Mean translational errors documented by pretreatment OSMS