ESTRO 36 Abstract Book

S149 ESTRO 36 _______________________________________________________________________________________________

(Figure1) .

Conclusion Three dimensional performance analysis showed that Cyberknife Synchrony® RMTS is successful in tumor tracking regardless of the amplitude of movement. This study is supported by TUBITAK 3001 project, project number 115S446 PV-0285 Using a surface scanner for positioning of pelvic patients - can X-ray images be omitted? J.B. Thomsen 1 , S. B.N. Biancardo 1 , S.H. Hattel 1 , L. Søndergaard Vinther Merkelsen 1 , B. Roche 1 , J.P. Bangsgaard 1 1 Rigshospitalet, Finsenscentret- Radiotherapy, Copenhagen, Denmark Purpose or Objective Reproducing the correct treatment position prior to radiotherapy is crucial for accurate dose delivery. The golden standard for positioning is X-ray based imaging with the drawback of exposing the patient to ionising radiation. More recently surface scanners using infrared light has been introduced to monitor the patient surface. We investigate a surface monitor system ”AlignRT” for positioning pelvic patients prior radiotherapy [Vision RT, www.visionrt.com ]. This is attractive in terms of saving time and reducing imaging dose to the patient. Even when acquiring daily X-ray images routinely, the ability to correct rotations using AlignRT is of value to limit Patients undergoing pelvic irradiation were positioned using the surface scanner. The body surface was extracted from the CT therapy scan acquired before radiotherapy and imported in the surface scanner software. With the patient on the couch it is possible to monitor the surface in the treatment region and the system displays the deviations from the CT therapy scan translational and rotational. We chose a region of interest (ROI) around the treatment region of about 20 cm in cranio-caudal direction and extending on both sides of the patient. Following positioning using the surface scanner, a cone-beam CT scan (CBCT) was acquired which makes a comparison between the positioning using the surface scanner and the CBCT possible. For the CBCT an automatic bone match was applied using commercial software (Varian inc., offline review) and visually inspected. In total, 105 fractions from 6 patients were analyzed and a paired T-test was applied to detect any significant differences between the two systems. Results In 50 of 105 fractions (48%) the difference between the two positioning procedures was larger than 5 mm in at least one direction. In 39 of 78 fractions (37%) the difference in rotations was larger than 3 degrees . In 71 of 105 fractions (68%) either a difference in translations was above 5mm or rotation above 3 degrees. In fig. 1 and fig. 2 the difference in translations and rotations are shown for all 105 fractions originating from 6 patients. For the translation there was a significant difference for vertical and longitudinal directions (P<0,001 for vertical, P<0,001 for longitudinal, P<0,9 for lateral). The difference in rotations were all significant (P<0,001 for pitch, P<0,05 for roll, P<0,01 for rotation around the vertical axis). repeated X-ray images. Material and Methods

PV-0284 3D Performance Analysis of Cyberknife Synchrony® Respiratory Tracking System M.C. Sahin 1 , P. Hurmuz 1 , M. Yeginer 1 , G. Yazici 1 , G. Ozyigit 1 1 Hacettepe University Faculty of Medicine, Radiation Oncology, Ankara, Turkey Purpose or Objective Tumor movement is a challenging issue for the precise delivery of radiation for thoracic tumors. The Synchrony respiratory motion tracking system (RMTS) of Cyberknife® robotic radiosurgery unit synchronizes radiation beam delivery with the respiration induced tumor motion. This study aims to investigate the performance of Synchrony RMTS for different movement widths using polymer gel dosimetry. To the best of our knowledge this is the first study to make the three dimensional performance analysis of Synchrony RMTS. Material and Methods The MultiPlan® treatment planning system (TPS) of Cyberknife® was used to deliver 4 Gy to a tumor of 1X1X1 cm 3 . BrainLab Gating lung phantom was used to simulate lung movements with three different amplitudes (1 cm, 2 cm and 3 cm). Three fiducials were inserted to the phantom for tracking. Radiochromic film and polymer gel dosimetry were used and measurements were compared with the dose distributions acquired from the TPS. The dose information of irradiated gel were read out using 1.5 T magnetic resonance imaging. The gamma index values were analysed using the Ashland FilmQA Pro 3.0 software for film dosimeters and Polygevero software for gel dosimeters using the 3mm/3% criteria. PolyGevero gamma index value of ≤1 is accepted as a passing criteria according to the literature. Results The mean 3 mm/3% gamma index values of film dosimetry were 92.6±1.94%, 91.0±4.00%, 90.3±2.04% for tumor motions of 1 cm, 2 cm and 3 cm, respectively (p<0.001). For polymer gel dosimetry, the mean gamma index values calculated over almost three million points were 0.56±0.10, 0.60±0.24 and 0.65±0.30 for tumor motions of 1 cm, 2 cm and 3 cm, respectively (p<0.001). Although the difference was statistically significant for 3 different amplitudes, the performance of the system was within the acceptance limits